diff --git a/crsn/src/asm/data/literal.rs b/crsn/src/asm/data/literal.rs
index 7bf98e5..54bc73d 100644
--- a/crsn/src/asm/data/literal.rs
+++ b/crsn/src/asm/data/literal.rs
@@ -103,25 +103,13 @@ impl From<String> for Label {
     }
 }
 
-/// Routine name
+/// Routine name, including the routine's arity
 #[derive(Debug, Clone, Eq, PartialEq, Hash)]
-pub struct RoutineName(pub String);
+pub struct RoutineName { pub name: String, pub arity: u8 }
 
 impl Display for RoutineName {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        write!(f, "{}", self.0)
-    }
-}
-
-impl From<&str> for RoutineName {
-    fn from(n: &str) -> Self {
-        Self(n.to_string())
-    }
-}
-
-impl From<String> for RoutineName {
-    fn from(n: String) -> Self {
-        Self(n)
+        write!(f, "{}/{}", self.name, self.arity)
     }
 }
 
diff --git a/crsn/src/asm/error.rs b/crsn/src/asm/error.rs
index 8ee60d4..53077b8 100644
--- a/crsn/src/asm/error.rs
+++ b/crsn/src/asm/error.rs
@@ -5,6 +5,7 @@ use thiserror::Error;
 use crate::asm::data::{Mask, Register};
 use crate::asm::data::literal::Label;
 use crate::asm::instr::Cond;
+use std::num::ParseIntError;
 
 /// csn_asm unified error type
 #[derive(Error, Debug)]
@@ -23,6 +24,12 @@ pub enum CrsnError {
     Other(#[from] anyhow::Error),
 }
 
+impl From<std::num::ParseIntError> for CrsnError {
+    fn from(e: ParseIntError) -> Self {
+        CrsnError::Other(anyhow::anyhow!(e))
+    }
+}
+
 /// Error from the assembler stage (after parsing S-expressions and basic validation)
 #[derive(Error, Debug)]
 pub enum AsmError {
diff --git a/crsn/src/asm/parse/parse_routine.rs b/crsn/src/asm/parse/parse_routine.rs
index c2a5be2..7ead555 100644
--- a/crsn/src/asm/parse/parse_routine.rs
+++ b/crsn/src/asm/parse/parse_routine.rs
@@ -8,11 +8,23 @@ use crate::asm::patches::SexpIsA;
 use crate::asm::parse::parse_data::parse_reg_alias;
 use crate::asm::data::Register;
 use crate::asm::data::literal::RoutineName;
+use crate::builtin::parse::parse_routine_name;
 
 pub fn parse_routine(mut toki: impl Iterator<Item=Sexp> + Clone, pcx: &ParserContext) -> Result<Box<dyn Flatten>, CrsnError> {
     let name = expect_string_atom(toki.next())?;
+    let mut name = parse_routine_name(name)?;
 
     let arg_tokens = TokenParser::new(toki.clone().take_while(|e| e.is_atom()).collect(), pcx);
+
+    // If arity is explicitly given, then either no named argument must be provided,
+    // or their count must match the arity. If no arity is given, then arity is determined
+    // by the number of named arguments.
+    if name.arity == 0 && arg_tokens.len() != 0 {
+        name.arity = arg_tokens.len() as u8;
+    } else if arg_tokens.len() != 0 && name.arity as usize != arg_tokens.len() {
+        return Err(CrsnError::Parse(format!("arity mismatch in routine {}", name.name).into()));
+    }
+
     let toki = toki.skip_while(|e| e.is_atom());
 
     {
@@ -41,7 +53,7 @@ pub fn parse_routine(mut toki: impl Iterator<Item=Sexp> + Clone, pcx: &ParserCon
     }
 
     return Ok(Box::new(Routine {
-        name: RoutineName(name),
+        name,
         body,
     }));
 }
diff --git a/crsn/src/builtin/parse.rs b/crsn/src/builtin/parse.rs
index 6880f48..aec0d67 100644
--- a/crsn/src/builtin/parse.rs
+++ b/crsn/src/builtin/parse.rs
@@ -120,7 +120,7 @@ impl CrsnExtension for BuiltinOps {
             }
 
             "call" => {
-                let dest = RoutineName(args.next_string()?);
+                let dest = RoutineName { name: args.next_string()?, arity: args.len() as u8 };
 
                 let mut call_args = vec![];
                 for t in args {
@@ -138,8 +138,8 @@ impl CrsnExtension for BuiltinOps {
             }
 
             "routine" => {
-                let dest = RoutineName(args.next_string()?);
-                BuiltinOp::Routine(dest)
+                let name = args.next_string()?;
+                BuiltinOp::Routine(parse_routine_name(name)?)
             }
 
             "sk" => {
@@ -227,3 +227,16 @@ impl CrsnExtension for BuiltinOps {
         })))
     }
 }
+
+pub(crate) fn parse_routine_name(name : String) -> Result<RoutineName, CrsnError> {
+    let (name, arity) = if let Some(n) = name.find('/') {
+        (
+            (&name[0..n]).to_string(),
+            (&name[(n+1)..]).parse::<u8>()?
+        )
+    } else {
+        (name, 0)
+    };
+
+    Ok(RoutineName { name, arity })
+}
diff --git a/examples/arities.csn b/examples/arities.csn
new file mode 100644
index 0000000..bc23999
--- /dev/null
+++ b/examples/arities.csn
@@ -0,0 +1,48 @@
+(
+    (call slash 1)
+    (cmp res0 1 (ne? (fault)))
+
+    (call named 2)
+    (cmp res0 2 (ne? (fault)))
+
+    (call both 3)
+    (cmp res0 3 (ne? (fault)))
+
+    (call slash 1 2)
+    (cmp res0 1 (ne? (fault)))
+    (cmp res1 2 (ne? (fault)))
+
+    (call named 2 3)
+    (cmp res0 2 (ne? (fault)))
+    (cmp res1 3 (ne? (fault)))
+
+    (call both 3 4)
+    (cmp res0 3 (ne? (fault)))
+    (cmp res1 4 (ne? (fault)))
+
+    (halt)
+
+    (proc slash/1
+        (ret arg0)
+    )
+
+    (proc slash/2
+        (ret arg0 arg1)
+    )
+
+    (proc named a
+        (ret a)
+    )
+
+    (proc named a b
+        (ret a b)
+    )
+
+    (proc both/1 a
+        (ret a)
+    )
+
+    (proc both/2 a b
+        (ret a b)
+    )
+)
diff --git a/examples/factorial.csn b/examples/factorial.csn
index 27073a1..3f92d35 100644
--- a/examples/factorial.csn
+++ b/examples/factorial.csn
@@ -3,7 +3,7 @@
     (ld r0 res0)
     (halt)
 
-    (proc fac
+    (proc fac/1
         (cmp arg0 2
             (eq? (ret 2)))
         (sub r0 arg0 1)