diff --git a/Cargo.lock b/Cargo.lock
index 9ec40a63..fd9ce8b6 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -61,6 +61,7 @@ dependencies = [
  "pallas-crypto",
  "pallas-primitives",
  "pallas-traverse",
+ "petgraph",
  "regex",
  "serde",
  "serde_json",
@@ -302,6 +303,12 @@ dependencies = [
  "instant",
 ]
 
+[[package]]
+name = "fixedbitset"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0ce7134b9999ecaf8bcd65542e436736ef32ddca1b3e06094cb6ec5755203b80"
+
 [[package]]
 name = "flat-rs"
 version = "0.0.21"
@@ -704,6 +711,16 @@ version = "0.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f9b0efd3ba03c3a409d44d60425f279ec442bcf0b9e63ff4e410da31c8b0f69f"
 
+[[package]]
+name = "petgraph"
+version = "0.6.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e6d5014253a1331579ce62aa67443b4a658c5e7dd03d4bc6d302b94474888143"
+dependencies = [
+ "fixedbitset",
+ "indexmap",
+]
+
 [[package]]
 name = "ppv-lite86"
 version = "0.2.16"
diff --git a/crates/cli/Cargo.toml b/crates/cli/Cargo.toml
index 9d95fe93..19e34dc7 100644
--- a/crates/cli/Cargo.toml
+++ b/crates/cli/Cargo.toml
@@ -27,3 +27,4 @@ ignore = "0.4.18"
 regex = "1.6.0"
 miette = { version = "5.3.0", features = ["fancy"] }
 thiserror = "1.0.37"
+petgraph = "0.6.2"
diff --git a/crates/cli/src/error.rs b/crates/cli/src/error.rs
index e56b64b1..ce7ae86e 100644
--- a/crates/cli/src/error.rs
+++ b/crates/cli/src/error.rs
@@ -10,8 +10,16 @@ use miette::{EyreContext, LabeledSpan, MietteHandlerOpts, RgbColors, SourceCode}
 #[allow(dead_code)]
 #[derive(thiserror::Error)]
 pub enum Error {
+    #[error("duplicate module {module}")]
+    DuplicateModule {
+        module: String,
+        first: PathBuf,
+        second: PathBuf,
+    },
     #[error("file operation failed")]
-    FileIo { path: PathBuf, error: io::Error },
+    FileIo { error: io::Error, path: PathBuf },
+    #[error("cyclical module imports")]
+    ImportCycle { modules: Vec<String> },
     #[error("failed to parse")]
     Parse {
         path: PathBuf,
@@ -48,32 +56,47 @@ impl Debug for Error {
 impl miette::Diagnostic for Error {
     fn code<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
         match self {
-            Error::Parse { .. } => Some(Box::new("aiken::parser".to_string())),
-            Error::FileIo { .. } => None,
-            Error::List(_) => None,
-        }
-    }
-
-    fn source_code(&self) -> Option<&dyn SourceCode> {
-        match self {
-            Error::Parse { src, .. } => Some(src),
-            Error::FileIo { .. } => None,
-            Error::List(_) => None,
-        }
-    }
-
-    fn labels(&self) -> Option<Box<dyn Iterator<Item = LabeledSpan> + '_>> {
-        match self {
-            Error::Parse { error, .. } => error.labels(),
+            Error::DuplicateModule { .. } => Some(Box::new("aiken::project::duplicate_module")),
             Error::FileIo { .. } => None,
+            Error::ImportCycle { .. } => Some(Box::new("aiken::project::cyclical_import")),
+            Error::Parse { .. } => Some(Box::new("aiken::parser")),
             Error::List(_) => None,
         }
     }
 
     fn help<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
         match self {
-            Error::Parse { error, .. } => error.kind.help(),
+            Error::DuplicateModule { first, second, .. } => Some(Box::new(format!(
+                "rename either {} or {}",
+                first.display(),
+                second.display()
+            ))),
             Error::FileIo { .. } => None,
+            Error::ImportCycle { modules } => Some(Box::new(format!(
+                "try moving the shared code to a separate module that the others can depend on\n- {}",
+                modules.join("\n- ")
+            ))),
+            Error::Parse { error, .. } => error.kind.help(),
+            Error::List(_) => None,
+        }
+    }
+
+    fn labels(&self) -> Option<Box<dyn Iterator<Item = LabeledSpan> + '_>> {
+        match self {
+            Error::DuplicateModule { .. } => None,
+            Error::FileIo { .. } => None,
+            Error::ImportCycle { .. } => None,
+            Error::Parse { error, .. } => error.labels(),
+            Error::List(_) => None,
+        }
+    }
+
+    fn source_code(&self) -> Option<&dyn SourceCode> {
+        match self {
+            Error::DuplicateModule { .. } => None,
+            Error::FileIo { .. } => None,
+            Error::ImportCycle { .. } => None,
+            Error::Parse { src, .. } => Some(src),
             Error::List(_) => None,
         }
     }
diff --git a/crates/cli/src/lib.rs b/crates/cli/src/lib.rs
index 37fc93f1..77ec66a1 100644
--- a/crates/cli/src/lib.rs
+++ b/crates/cli/src/lib.rs
@@ -1,3 +1,4 @@
 pub mod config;
 pub mod error;
+pub mod module;
 pub mod project;
diff --git a/crates/cli/src/module.rs b/crates/cli/src/module.rs
new file mode 100644
index 00000000..cd814f9d
--- /dev/null
+++ b/crates/cli/src/module.rs
@@ -0,0 +1,151 @@
+use std::{
+    collections::{HashMap, HashSet},
+    ops::Deref,
+    path::PathBuf,
+};
+
+use aiken_lang::ast::{ModuleKind, UntypedModule};
+use petgraph::{algo, graph::NodeIndex, Direction, Graph};
+
+use crate::error::Error;
+
+#[derive(Debug)]
+#[allow(dead_code)]
+pub struct ParsedModule {
+    pub path: PathBuf,
+    pub name: String,
+    pub code: String,
+    pub kind: ModuleKind,
+    pub package: String,
+    pub ast: UntypedModule,
+    // extra: ModuleExtra,
+}
+
+impl ParsedModule {
+    pub fn deps_for_graph(&self) -> (String, Vec<String>) {
+        let name = self.name.clone();
+
+        let deps: Vec<_> = self
+            .ast
+            .dependencies()
+            .into_iter()
+            .map(|(dep, _span)| dep)
+            .collect();
+
+        (name, deps)
+    }
+}
+
+pub struct ParsedModules(HashMap<String, ParsedModule>);
+
+impl ParsedModules {
+    pub fn sequence(&self) -> Result<Vec<String>, Error> {
+        let inputs = self
+            .0
+            .values()
+            .map(|m| m.deps_for_graph())
+            .collect::<Vec<(String, Vec<String>)>>();
+
+        let capacity = inputs.len();
+
+        let mut graph = Graph::<(), ()>::with_capacity(capacity, capacity * 5);
+
+        // TODO: maybe use a bimap?
+        let mut indices = HashMap::with_capacity(capacity);
+        let mut values = HashMap::with_capacity(capacity);
+
+        for (value, _) in &inputs {
+            let index = graph.add_node(());
+
+            indices.insert(value.clone(), index);
+
+            values.insert(index, value.clone());
+        }
+
+        for (value, deps) in inputs {
+            if let Some(from_index) = indices.get(&value) {
+                let deps = deps.into_iter().filter_map(|dep| indices.get(&dep));
+
+                for to_index in deps {
+                    graph.add_edge(*from_index, *to_index, ());
+                }
+            }
+        }
+
+        match algo::toposort(&graph, None) {
+            Ok(sequence) => {
+                let sequence = sequence
+                    .iter()
+                    .filter_map(|i| values.remove(i))
+                    .rev()
+                    .collect();
+
+                Ok(sequence)
+            }
+            Err(cycle) => {
+                let origin = cycle.node_id();
+
+                let mut path = vec![];
+
+                find_cycle(origin, origin, &graph, &mut path, &mut HashSet::new());
+
+                let modules = path
+                    .iter()
+                    .filter_map(|index| values.remove(index))
+                    .collect();
+
+                Err(Error::ImportCycle { modules })
+            }
+        }
+    }
+}
+
+impl From<HashMap<String, ParsedModule>> for ParsedModules {
+    fn from(parsed_modules: HashMap<String, ParsedModule>) -> Self {
+        ParsedModules(parsed_modules)
+    }
+}
+
+impl From<ParsedModules> for HashMap<String, ParsedModule> {
+    fn from(parsed_modules: ParsedModules) -> Self {
+        parsed_modules.0
+    }
+}
+
+impl Deref for ParsedModules {
+    type Target = HashMap<String, ParsedModule>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+fn find_cycle(
+    origin: NodeIndex,
+    parent: NodeIndex,
+    graph: &petgraph::Graph<(), ()>,
+    path: &mut Vec<NodeIndex>,
+    seen: &mut HashSet<NodeIndex>,
+) -> bool {
+    seen.insert(parent);
+
+    for node in graph.neighbors_directed(parent, Direction::Outgoing) {
+        if node == origin {
+            path.push(node);
+
+            return true;
+        }
+
+        if seen.contains(&node) {
+            continue;
+        }
+
+        if find_cycle(origin, node, graph, path, seen) {
+            path.push(node);
+
+            return true;
+        }
+    }
+
+    false
+}
diff --git a/crates/cli/src/project.rs b/crates/cli/src/project.rs
index 059f3e43..b1539406 100644
--- a/crates/cli/src/project.rs
+++ b/crates/cli/src/project.rs
@@ -4,9 +4,13 @@ use std::{
     path::{Path, PathBuf},
 };
 
-use aiken_lang::ast::{ModuleKind, UntypedModule};
+use aiken_lang::ast::ModuleKind;
 
-use crate::{config::Config, error::Error};
+use crate::{
+    config::Config,
+    error::Error,
+    module::{ParsedModule, ParsedModules},
+};
 
 #[derive(Debug)]
 pub struct Source {
@@ -16,22 +20,11 @@ pub struct Source {
     pub kind: ModuleKind,
 }
 
-#[derive(Debug)]
-#[allow(dead_code)]
-struct ParsedModule {
-    path: PathBuf,
-    name: String,
-    code: String,
-    kind: ModuleKind,
-    package: String,
-    ast: UntypedModule,
-    // extra: ModuleExtra,
-}
-
 pub struct Project {
     config: Config,
     root: PathBuf,
     sources: Vec<Source>,
+    defined_modules: HashMap<String, PathBuf>,
 }
 
 impl Project {
@@ -40,18 +33,21 @@ impl Project {
             config,
             root,
             sources: vec![],
+            defined_modules: HashMap::new(),
         }
     }
 
     pub fn build(&mut self) -> Result<(), Error> {
         self.read_source_files()?;
 
-        self.parse_sources()?;
+        let parsed_modules = self.parse_sources()?;
+
+        let processing_sequence = parsed_modules.sequence()?;
 
         Ok(())
     }
 
-    fn parse_sources(&mut self) -> Result<HashMap<String, ParsedModule>, Error> {
+    fn parse_sources(&mut self) -> Result<ParsedModules, Error> {
         let mut errors = Vec::new();
         let mut parsed_modules = HashMap::with_capacity(self.sources.len());
 
@@ -73,10 +69,21 @@ impl Project {
                         code,
                         name,
                         path,
-                        package: "".to_string(),
+                        package: self.config.name.clone(),
                     };
 
-                    let _ = parsed_modules.insert(module.name.clone(), module);
+                    if let Some(first) = self
+                        .defined_modules
+                        .insert(module.name.clone(), module.path.clone())
+                    {
+                        return Err(Error::DuplicateModule {
+                            module: module.name.clone(),
+                            first,
+                            second: module.path,
+                        });
+                    }
+
+                    parsed_modules.insert(module.name.clone(), module);
                 }
                 Err(errs) => {
                     for error in errs {
@@ -91,7 +98,7 @@ impl Project {
         }
 
         if errors.is_empty() {
-            Ok(parsed_modules)
+            Ok(parsed_modules.into())
         } else {
             Err(Error::List(errors))
         }
diff --git a/crates/lang/src/ast.rs b/crates/lang/src/ast.rs
index 99cd267b..94118902 100644
--- a/crates/lang/src/ast.rs
+++ b/crates/lang/src/ast.rs
@@ -37,6 +37,31 @@ pub struct Module<Info, Definitions> {
     pub kind: ModuleKind,
 }
 
+impl UntypedModule {
+    pub fn dependencies(&self) -> Vec<(String, Span)> {
+        self.definitions()
+            .flat_map(|def| {
+                if let Definition::Use {
+                    location, module, ..
+                } = def
+                {
+                    Some((module.join("/"), *location))
+                } else {
+                    None
+                }
+            })
+            .collect()
+    }
+
+    pub fn definitions(&self) -> impl Iterator<Item = &UntypedDefinition> {
+        self.definitions.iter()
+    }
+
+    pub fn into_definitions(self) -> impl Iterator<Item = UntypedDefinition> {
+        self.definitions.into_iter()
+    }
+}
+
 pub type TypedDefinition = Definition<Arc<Type>, TypedExpr, String, String>;
 pub type UntypedDefinition = Definition<(), UntypedExpr, (), ()>;
 
@@ -500,7 +525,7 @@ pub struct Span {
 
 impl From<Span> for miette::SourceSpan {
     fn from(span: Span) -> Self {
-        Self::new(span.start.into(), span.end.into())
+        Self::new(span.start.into(), (span.end - span.start).into())
     }
 }