diff --git a/crates/flat/src/decode.rs b/crates/flat/src/decode.rs
index 92031d56..e74ecc7f 100644
--- a/crates/flat/src/decode.rs
+++ b/crates/flat/src/decode.rs
@@ -17,6 +17,24 @@ impl Decode<'_> for Vec<u8> {
     }
 }
 
+impl Decode<'_> for u8 {
+    fn decode(d: &mut Decoder) -> Result<Self, String> {
+        d.u8()
+    }
+}
+
+impl Decode<'_> for isize {
+    fn decode(d: &mut Decoder) -> Result<Self, String> {
+        d.integer()
+    }
+}
+
+impl Decode<'_> for usize {
+    fn decode(d: &mut Decoder) -> Result<Self, String> {
+        d.word()
+    }
+}
+
 impl Decode<'_> for char {
     fn decode(d: &mut Decoder) -> Result<Self, String> {
         d.char()
@@ -31,7 +49,6 @@ impl Decode<'_> for String {
 
 impl Decode<'_> for bool {
     fn decode(d: &mut Decoder) -> Result<bool, String> {
-        let b = d.bool();
-        Ok(b)
+        d.bool()
     }
 }
diff --git a/crates/flat/src/decoder.rs b/crates/flat/src/decoder.rs
index 95ca1362..e05cb621 100644
--- a/crates/flat/src/decoder.rs
+++ b/crates/flat/src/decoder.rs
@@ -1,4 +1,4 @@
-use crate::decode::Decode;
+use crate::{decode::Decode, zigzag};
 
 pub struct Decoder<'b> {
     buffer: &'b [u8],
@@ -19,11 +19,19 @@ impl<'b> Decoder<'b> {
         T::decode(self)
     }
 
-    pub fn bool(&mut self) -> bool {
+    pub fn integer(&mut self) -> Result<isize, String> {
+        Ok(zigzag::to_isize(self.word()?))
+    }
+
+    pub fn bool(&mut self) -> Result<bool, String> {
         let current_byte = self.buffer[self.pos];
         let b = 0 != (current_byte & (128 >> self.used_bits));
         self.increment_buffer_by_bit();
-        b
+        Ok(b)
+    }
+
+    pub fn u8(&mut self) -> Result<u8, String> {
+        self.bits8(8)
     }
 
     pub fn bytes(&mut self) -> Result<Vec<u8>, String> {
@@ -48,7 +56,7 @@ impl<'b> Decoder<'b> {
         Ok(())
     }
 
-    fn word(&mut self) -> Result<usize, String> {
+    pub fn word(&mut self) -> Result<usize, String> {
         let mut leading_bit = 1;
         let mut final_word: usize = 0;
         let mut shl: usize = 0;
@@ -63,6 +71,17 @@ impl<'b> Decoder<'b> {
         Ok(final_word)
     }
 
+    pub fn decode_list_with<T: Decode<'b>>(
+        &mut self,
+        decoder_func: for<'r> fn(&'r mut Decoder) -> Result<T, String>,
+    ) -> Result<Vec<T>, String> {
+        let mut vec_array: Vec<T> = Vec::new();
+        while self.bit()? {
+            vec_array.push(decoder_func(self)?)
+        }
+        Ok(vec_array)
+    }
+
     fn zero(&mut self) -> Result<bool, String> {
         let current_bit = self.bit()?;
         Ok(!current_bit)
@@ -102,7 +121,7 @@ impl<'b> Decoder<'b> {
                 "Decoder.bits8: incorrect value of num_bits - must be less than 9".to_string(),
             );
         }
-        self.ensure_bytes(num_bits)?;
+        self.ensure_bits(num_bits)?;
         let unused_bits = 8 - self.used_bits as usize;
         let leading_zeroes = 8 - num_bits;
         let r = (self.buffer[self.pos] << self.used_bits as usize) >> leading_zeroes;
@@ -112,12 +131,12 @@ impl<'b> Decoder<'b> {
         } else {
             r
         };
-        self.drop_bits(8);
+        self.drop_bits(num_bits);
         Ok(x)
     }
 
     fn ensure_bytes(&mut self, required_bytes: usize) -> Result<(), String> {
-        if required_bytes > self.buffer.len() - self.pos {
+        if required_bytes as isize > self.buffer.len() as isize - self.pos as isize {
             return Err(format!(
                 "DecoderState: Not enough data available: {:#?} - required bytes {}",
                 self.buffer, required_bytes
@@ -126,6 +145,18 @@ impl<'b> Decoder<'b> {
         Ok(())
     }
 
+    fn ensure_bits(&mut self, required_bits: usize) -> Result<(), String> {
+        if required_bits as isize
+            > (self.buffer.len() as isize - self.pos as isize) * 8 - self.used_bits as isize
+        {
+            return Err(format!(
+                "DecoderState: Not enough data available: {:#?} - required bits {}",
+                self.buffer, required_bits
+            ));
+        }
+        Ok(())
+    }
+
     fn drop_bits(&mut self, num_bits: usize) {
         let all_used_bits = num_bits as i64 + self.used_bits;
         self.used_bits = all_used_bits % 8;
diff --git a/crates/uplc/src/builtins.rs b/crates/uplc/src/builtins.rs
index 9231c494..638ed6f1 100644
--- a/crates/uplc/src/builtins.rs
+++ b/crates/uplc/src/builtins.rs
@@ -78,3 +78,115 @@ pub enum DefaultFunction {
     MkNilData = 52,
     MkNilPairData = 53,
 }
+
+impl TryFrom<u8> for DefaultFunction {
+    fn try_from(v: u8) -> Result<Self, Self::Error> {
+        match v {
+            v if v == DefaultFunction::AddInteger as u8 => Ok(DefaultFunction::AddInteger),
+            v if v == DefaultFunction::SubtractInteger as u8 => {
+                Ok(DefaultFunction::SubtractInteger)
+            }
+            v if v == DefaultFunction::MultiplyInteger as u8 => {
+                Ok(DefaultFunction::MultiplyInteger)
+            }
+            v if v == DefaultFunction::DivideInteger as u8 => Ok(DefaultFunction::DivideInteger),
+            v if v == DefaultFunction::QuotientInteger as u8 => {
+                Ok(DefaultFunction::QuotientInteger)
+            }
+            v if v == DefaultFunction::RemainderInteger as u8 => {
+                Ok(DefaultFunction::RemainderInteger)
+            }
+            v if v == DefaultFunction::ModInteger as u8 => Ok(DefaultFunction::ModInteger),
+            v if v == DefaultFunction::EqualsInteger as u8 => Ok(DefaultFunction::EqualsInteger),
+            v if v == DefaultFunction::LessThanInteger as u8 => {
+                Ok(DefaultFunction::LessThanInteger)
+            }
+            v if v == DefaultFunction::LessThanEqualsInteger as u8 => {
+                Ok(DefaultFunction::LessThanEqualsInteger)
+            }
+            // ByteString functions
+            v if v == DefaultFunction::AppendByteString as u8 => {
+                Ok(DefaultFunction::AppendByteString)
+            }
+            v if v == DefaultFunction::ConsByteString as u8 => Ok(DefaultFunction::ConsByteString),
+            v if v == DefaultFunction::SliceByteString as u8 => {
+                Ok(DefaultFunction::SliceByteString)
+            }
+            v if v == DefaultFunction::LengthOfByteString as u8 => {
+                Ok(DefaultFunction::LengthOfByteString)
+            }
+            v if v == DefaultFunction::IndexByteString as u8 => {
+                Ok(DefaultFunction::IndexByteString)
+            }
+            v if v == DefaultFunction::EqualsByteString as u8 => {
+                Ok(DefaultFunction::EqualsByteString)
+            }
+            v if v == DefaultFunction::LessThanByteString as u8 => {
+                Ok(DefaultFunction::LessThanByteString)
+            }
+            v if v == DefaultFunction::LessThanEqualsByteString as u8 => {
+                Ok(DefaultFunction::LessThanEqualsByteString)
+            }
+            // Cryptography and hash functions
+            v if v == DefaultFunction::Sha2_256 as u8 => Ok(DefaultFunction::Sha2_256),
+            v if v == DefaultFunction::Sha3_256 as u8 => Ok(DefaultFunction::Sha3_256),
+            v if v == DefaultFunction::Blake2b_256 as u8 => Ok(DefaultFunction::Blake2b_256),
+            v if v == DefaultFunction::VerifySignature as u8 => {
+                Ok(DefaultFunction::VerifySignature)
+            }
+            v if v == DefaultFunction::VerifyEcdsaSecp256k1Signature as u8 => {
+                Ok(DefaultFunction::VerifyEcdsaSecp256k1Signature)
+            }
+            v if v == DefaultFunction::VerifySchnorrSecp256k1Signature as u8 => {
+                Ok(DefaultFunction::VerifySchnorrSecp256k1Signature)
+            }
+            // String functions
+            v if v == DefaultFunction::AppendString as u8 => Ok(DefaultFunction::AppendString),
+            v if v == DefaultFunction::EqualsString as u8 => Ok(DefaultFunction::EqualsString),
+            v if v == DefaultFunction::EncodeUtf8 as u8 => Ok(DefaultFunction::EncodeUtf8),
+            v if v == DefaultFunction::DecodeUtf8 as u8 => Ok(DefaultFunction::DecodeUtf8),
+            // Bool function
+            v if v == DefaultFunction::IfThenElse as u8 => Ok(DefaultFunction::IfThenElse),
+            // Unit function
+            v if v == DefaultFunction::ChooseUnit as u8 => Ok(DefaultFunction::ChooseUnit),
+            // Tracing function
+            v if v == DefaultFunction::Trace as u8 => Ok(DefaultFunction::Trace),
+            // Pairs functions
+            v if v == DefaultFunction::FstPair as u8 => Ok(DefaultFunction::FstPair),
+            v if v == DefaultFunction::SndPair as u8 => Ok(DefaultFunction::SndPair),
+            // List functions
+            v if v == DefaultFunction::ChooseList as u8 => Ok(DefaultFunction::ChooseList),
+            v if v == DefaultFunction::MkCons as u8 => Ok(DefaultFunction::MkCons),
+            v if v == DefaultFunction::HeadList as u8 => Ok(DefaultFunction::HeadList),
+            v if v == DefaultFunction::TailList as u8 => Ok(DefaultFunction::TailList),
+            v if v == DefaultFunction::NullList as u8 => Ok(DefaultFunction::NullList),
+            // Data functions
+            // It is convenient to have a "choosing" function for a data type that has more than two
+            // constructors to get pattern matching over it and we may end up having multiple such data
+            // types, hence we include the name of the data type as a suffix.
+            v if v == DefaultFunction::ChooseData as u8 => Ok(DefaultFunction::ChooseData),
+            v if v == DefaultFunction::ConstrData as u8 => Ok(DefaultFunction::ConstrData),
+            v if v == DefaultFunction::MapData as u8 => Ok(DefaultFunction::MapData),
+            v if v == DefaultFunction::ListData as u8 => Ok(DefaultFunction::ListData),
+            v if v == DefaultFunction::IData as u8 => Ok(DefaultFunction::IData),
+            v if v == DefaultFunction::BData as u8 => Ok(DefaultFunction::BData),
+            v if v == DefaultFunction::UnConstrData as u8 => Ok(DefaultFunction::UnConstrData),
+            v if v == DefaultFunction::UnMapData as u8 => Ok(DefaultFunction::UnMapData),
+            v if v == DefaultFunction::UnListData as u8 => Ok(DefaultFunction::UnListData),
+            v if v == DefaultFunction::UnIData as u8 => Ok(DefaultFunction::UnIData),
+            v if v == DefaultFunction::UnBData as u8 => Ok(DefaultFunction::UnBData),
+            v if v == DefaultFunction::EqualsData as u8 => Ok(DefaultFunction::EqualsData),
+            v if v == DefaultFunction::SerialiseData as u8 => Ok(DefaultFunction::SerialiseData),
+            // Misc constructors
+            // Constructors that we need for constructing e.g. Data. Polymorphic builtin
+            // constructors are often problematic (See note [Representable built-in
+            // functions over polymorphic built-in types])
+            v if v == DefaultFunction::MkPairData as u8 => Ok(DefaultFunction::MkPairData),
+            v if v == DefaultFunction::MkNilData as u8 => Ok(DefaultFunction::MkNilData),
+            v if v == DefaultFunction::MkNilPairData as u8 => Ok(DefaultFunction::MkNilPairData),
+            _ => Err("Default Function not found".to_string()),
+        }
+    }
+
+    type Error = String;
+}
diff --git a/crates/uplc/src/flat.rs b/crates/uplc/src/flat.rs
index 5fc020f9..ccce887c 100644
--- a/crates/uplc/src/flat.rs
+++ b/crates/uplc/src/flat.rs
@@ -48,8 +48,10 @@ impl Encode for Program {
 }
 
 impl<'b> Decode<'b> for Program {
-    fn decode(_d: &mut Decoder) -> Result<Self, String> {
-        todo!()
+    fn decode(d: &mut Decoder) -> Result<Self, String> {
+        let version = (usize::decode(d)?, usize::decode(d)?, usize::decode(d)?);
+        let term = Term::decode(d)?;
+        Ok(Program { version, term })
     }
 }
 
@@ -104,8 +106,22 @@ impl Encode for Term {
 }
 
 impl<'b> Decode<'b> for Term {
-    fn decode(_d: &mut Decoder) -> Result<Self, String> {
-        todo!()
+    fn decode(d: &mut Decoder) -> Result<Self, String> {
+        match decode_term_tag(d)? {
+            0 => Ok(Term::Var(String::decode(d)?)),
+            1 => Ok(Term::Delay(Box::new(Term::decode(d)?))),
+            2 => todo!(),
+            3 => Ok(Term::Apply {
+                function: Box::new(Term::decode(d)?),
+                argument: Box::new(Term::decode(d)?),
+            }),
+            // Need size limit for Constant
+            4 => Ok(Term::Constant(Constant::decode(d)?)),
+            5 => Ok(Term::Force(Box::new(Term::decode(d)?))),
+            6 => Ok(Term::Error),
+            7 => Ok(Term::Builtin(DefaultFunction::decode(d)?)),
+            x => Err(format!("Unknown term constructor tag: {}", x)),
+        }
     }
 }
 
@@ -146,8 +162,17 @@ impl Encode for &Constant {
 }
 
 impl<'b> Decode<'b> for Constant {
-    fn decode(_d: &mut Decoder) -> Result<Self, String> {
-        todo!()
+    fn decode(d: &mut Decoder) -> Result<Self, String> {
+        match decode_constant(d)? {
+            0 => Ok(Constant::Integer(isize::decode(d)?)),
+            1 => Ok(Constant::ByteString(Vec::<u8>::decode(d)?)),
+            2 => Ok(Constant::String(
+                String::from_utf8(Vec::<u8>::decode(d)?).unwrap(),
+            )),
+            3 => Ok(Constant::Unit),
+            4 => Ok(Constant::Bool(bool::decode(d)?)),
+            x => Err(format!("Unknown constant constructor tag: {}", x)),
+        }
     }
 }
 
@@ -160,8 +185,9 @@ impl Encode for DefaultFunction {
 }
 
 impl<'b> Decode<'b> for DefaultFunction {
-    fn decode(_d: &mut Decoder) -> Result<Self, String> {
-        todo!()
+    fn decode(d: &mut Decoder) -> Result<Self, String> {
+        let builtin_tag = d.bits8(BUILTIN_TAG_WIDTH as usize)?;
+        builtin_tag.try_into()
     }
 }
 
@@ -169,6 +195,10 @@ fn encode_term_tag(tag: u8, e: &mut Encoder) -> Result<(), String> {
     safe_encode_bits(TERM_TAG_WIDTH, tag, e)
 }
 
+fn decode_term_tag(d: &mut Decoder) -> Result<u8, String> {
+    d.bits8(TERM_TAG_WIDTH as usize)
+}
+
 fn safe_encode_bits(num_bits: u32, byte: u8, e: &mut Encoder) -> Result<(), String> {
     if 2_u8.pow(num_bits) < byte {
         Err(format!(
@@ -185,10 +215,26 @@ pub fn encode_constant(tag: u8, e: &mut Encoder) -> Result<(), String> {
     e.encode_list_with(encode_constant_tag, [tag].to_vec())
 }
 
+pub fn decode_constant(d: &mut Decoder) -> Result<u8, String> {
+    let u8_list = d.decode_list_with(decode_constant_tag)?;
+    if u8_list.len() > 1 {
+        Err(
+            "Improper encoding on constant tag. Should be list of one item encoded in 4 bits"
+                .to_string(),
+        )
+    } else {
+        Ok(u8_list[0])
+    }
+}
+
 pub fn encode_constant_tag(tag: u8, e: &mut Encoder) -> Result<(), String> {
     safe_encode_bits(CONST_TAG_WIDTH, tag, e)
 }
 
+pub fn decode_constant_tag(d: &mut Decoder) -> Result<u8, String> {
+    d.bits8(CONST_TAG_WIDTH as usize)
+}
+
 #[cfg(test)]
 mod test {
     use super::{Constant, Program, Term};