diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index 7617380..1bb7425 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -7,6 +7,7 @@ use crate::Context;
 use crate::emit::Instruction;
 use crate::emit::VMInstructions;
 use crate::emit::VariableSlot;
+use crate::functions::FunctionMap;
 use crate::input::NomoInput;
 use crate::value::NomoValue;
 
@@ -69,7 +70,11 @@ impl Scope {
     clippy::unnecessary_to_owned,
     reason = "We cannot do the suggested way as the lifetimes would not match up"
 )]
-pub fn execute(vm: &VMInstructions, global_context: &Context) -> Result<String, EvaluationError> {
+pub fn execute(
+    available_functions: &FunctionMap,
+    vm: &VMInstructions,
+    global_context: &Context,
+) -> Result<String, EvaluationError> {
     let mut output = String::new();
 
     let mut scopes = Scope {
@@ -208,7 +213,17 @@ pub fn execute(vm: &VMInstructions, global_context: &Context) -> Result<String,
 
                 scopes.insert_into_slot(*result_slot, result.unwrap());
             }
-            Instruction::FunctionCall { .. } => todo!(),
+            Instruction::FunctionCall { name, args, slot } => {
+                let args = args.iter().map(|slot| scopes.get(slot)).cloned().collect();
+
+                let value = available_functions
+                    .get(name.as_str())
+                    .unwrap()
+                    .call(args)
+                    .unwrap();
+
+                scopes.insert_into_slot(*slot, value);
+            }
         }
 
         ip += 1;
@@ -221,6 +236,8 @@ pub fn execute(vm: &VMInstructions, global_context: &Context) -> Result<String,
 mod tests {
     use crate::Context;
     use crate::eval::execute;
+    use crate::functions::FunctionMap;
+    use crate::functions::NomoFunctionError;
 
     #[test]
     fn check_simple_variable_interpolation() {
@@ -234,7 +251,7 @@ mod tests {
 
         let mut context = Context::new();
         context.insert("world", "World");
-        let output = execute(&emit, &context);
+        let output = execute(&FunctionMap::default(), &emit, &context);
 
         insta::assert_debug_snapshot!(output, @r#"
         Ok(
@@ -242,4 +259,31 @@ mod tests {
         )
         "#);
     }
+
+    #[test]
+    fn check_method_call() {
+        let input = "Hello {{= foo(world) }}";
+
+        let parsed = crate::parser::parse(input.into()).unwrap();
+
+        let ast = crate::ast::parse(parsed.tokens()).unwrap();
+
+        let emit = crate::emit::emit_machine(ast);
+
+        let mut context = Context::new();
+        context.insert("world", "World");
+        let mut function_map = FunctionMap::default();
+
+        function_map.register("foo", |arg: String| -> Result<String, NomoFunctionError> {
+            Ok(arg.to_uppercase())
+        });
+
+        let output = execute(&function_map, &emit, &context);
+
+        insta::assert_debug_snapshot!(output, @r#"
+        Ok(
+            "Hello WORLD",
+        )
+        "#);
+    }
 }
diff --git a/src/functions.rs b/src/functions.rs
new file mode 100644
index 0000000..c15031c
--- /dev/null
+++ b/src/functions.rs
@@ -0,0 +1,175 @@
+use std::any::Any;
+use std::collections::HashMap;
+
+use displaydoc::Display;
+use thiserror::Error;
+
+use crate::NomoValueError;
+use crate::value::NomoValue;
+
+#[derive(Debug, Error, Display)]
+pub enum NomoFunctionError {
+    /// Received {received} arguments, but this function only takes {expected}
+    WrongArgumentCount { received: usize, expected: usize },
+
+    /// The argument at this position is of the wrong type
+    InvalidArgumentType { index: usize },
+
+    /// A user-provided error
+    #[error(transparent)]
+    CustomError {
+        custom: Box<dyn std::error::Error + Send + Sync>,
+    },
+}
+
+pub trait NomoFunction<T>: 'static + Send + Sync {
+    fn call(&self, args: Vec<NomoValue>) -> Result<NomoValue, NomoFunctionError>;
+}
+
+#[derive(Default)]
+pub struct FunctionMap {
+    funcs: HashMap<String, ErasedNomoFunction>,
+}
+
+impl FunctionMap {
+    pub fn register<NF: NomoFunction<T>, T>(&mut self, name: impl Into<String>, func: NF) {
+        self.funcs
+            .insert(name.into(), ErasedNomoFunction::erase(func));
+    }
+
+    pub fn get(&self, name: impl AsRef<str>) -> Option<&ErasedNomoFunction> {
+        self.funcs.get(name.as_ref())
+    }
+}
+
+pub struct ErasedNomoFunction {
+    func: Box<dyn Any + Send + Sync>,
+    call_fn: fn(&dyn Any, Vec<NomoValue>) -> Result<NomoValue, NomoFunctionError>,
+}
+
+impl ErasedNomoFunction {
+    pub fn call(&self, args: Vec<NomoValue>) -> Result<NomoValue, NomoFunctionError> {
+        (self.call_fn)(&*self.func, args)
+    }
+
+    pub fn erase<NF, T>(f: NF) -> ErasedNomoFunction
+    where
+        NF: NomoFunction<T>,
+    {
+        ErasedNomoFunction {
+            func: Box::new(f),
+            call_fn: |nf, args| {
+                let nf: &NF = nf.downcast_ref().unwrap();
+
+                nf.call(args)
+            },
+        }
+    }
+}
+
+#[rustfmt::skip]
+macro_rules! for_all_tuples {
+    ($name:ident) => {
+        $name!([], T1);
+        $name!([T1], T2);
+        $name!([T1, T2], T3);
+        $name!([T1, T2, T3], T4);
+        $name!([T1, T2, T3, T4], T5);
+        $name!([T1, T2, T3, T4, T5], T6);
+        $name!([T1, T2, T3, T4, T5, T6], T7);
+        $name!([T1, T2, T3, T4, T5, T6, T7], T8);
+        $name!([T1, T2, T3, T4, T5, T6, T7, T8], T9);
+        $name!([T1, T2, T3, T4, T5, T6, T7, T8, T9], T10);
+        $name!([T1, T2, T3, T4, T5, T6, T7, T8, T9, T10], T11);
+        $name!([T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11], T12);
+        $name!([T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12], T13);
+        $name!([T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13], T14);
+        $name!([T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14], T15);
+        $name!([T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15], T16);
+    };
+}
+
+macro_rules! impl_nomo_function {
+    ([$($ty:ident),*], $last:ident) => {
+        impl<F, Res, Error, $($ty,)* $last> NomoFunction<($($ty,)* $last,)> for F
+        where
+            F: Fn($($ty,)* $last,) -> Result<Res, Error>,
+            F: Send + Sync + 'static,
+            Res: Into<NomoValue>,
+            Error: Into<NomoFunctionError>,
+            $( $ty: TryFrom<NomoValue, Error = NomoValueError>, )*
+            $last: TryFrom<NomoValue, Error = NomoValueError>,
+        {
+            fn call(&self, args: Vec<NomoValue>) -> Result<NomoValue, NomoFunctionError> {
+                let arg_count = args.len();
+
+                let total_count = 1 $(+ impl_nomo_function!(count $ty))*;
+
+                if arg_count != total_count {
+                    return Err(NomoFunctionError::WrongArgumentCount {
+                        expected: total_count,
+                        received: arg_count,
+                    });
+                }
+
+                let mut args = args.into_iter();
+
+                #[allow(unused_mut)]
+                let mut idx = 0;
+
+                let val = (self)(
+                        $({
+                            let val = $ty::try_from(args.next().unwrap()).map_err(|_| NomoFunctionError::InvalidArgumentType { index: idx })?;
+                            idx += 1;
+                            val
+                        },)*
+                        $last::try_from(args.next().unwrap()).map_err(|_| NomoFunctionError::InvalidArgumentType { index: idx })?,
+                    );
+
+                val.map(Into::into).map_err(Into::into)
+            }
+        }
+    };
+
+    (count $ty:ident) => {
+        1
+    }
+}
+
+for_all_tuples!(impl_nomo_function);
+
+impl<F, Res, Error> NomoFunction<()> for F
+where
+    F: Fn() -> Result<Res, Error> + Send + Sync + 'static,
+    Res: Into<NomoValue>,
+    Error: Into<NomoFunctionError>,
+{
+    fn call(&self, args: Vec<NomoValue>) -> Result<NomoValue, NomoFunctionError> {
+        let arg_count = args.len();
+
+        if arg_count != 0 {
+            return Err(NomoFunctionError::WrongArgumentCount {
+                received: arg_count,
+                expected: 0,
+            });
+        }
+
+        let val = (self)();
+
+        val.map(Into::into).map_err(Into::into)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::functions::ErasedNomoFunction;
+    use crate::functions::NomoFunctionError;
+
+    #[expect(dead_code, reason = "This is a compile test")]
+    fn check_various_methods() {
+        ErasedNomoFunction::erase(|name: String| -> Result<String, NomoFunctionError> { Ok(name) });
+        ErasedNomoFunction::erase(|left: u64, right: u64| -> Result<u64, NomoFunctionError> {
+            Ok(left + right)
+        });
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 8d177ce..c9b8222 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -4,6 +4,7 @@ use displaydoc::Display;
 use thiserror::Error;
 
 use crate::emit::VMInstructions;
+use crate::functions::FunctionMap;
 use crate::input::NomoInput;
 use crate::value::NomoValue;
 use crate::value::NomoValueError;
@@ -11,6 +12,7 @@ use crate::value::NomoValueError;
 pub mod ast;
 pub mod emit;
 pub mod eval;
+pub mod functions;
 pub mod input;
 pub mod parser;
 pub mod value;
@@ -40,6 +42,7 @@ pub enum NomoError {
 
 pub struct Nomo {
     templates: HashMap<String, Template>,
+    function_map: FunctionMap,
 }
 
 impl Default for Nomo {
@@ -52,6 +55,7 @@ impl Nomo {
     pub fn new() -> Nomo {
         Nomo {
             templates: HashMap::new(),
+            function_map: FunctionMap::default(),
         }
     }
 
@@ -78,7 +82,7 @@ impl Nomo {
             .get(name)
             .ok_or_else(|| NomoError::UnknownTemplate(name.to_string()))?;
 
-        let res = eval::execute(&template.instructions, ctx)?;
+        let res = eval::execute(&self.function_map, &template.instructions, ctx)?;
 
         Ok(res)
     }
diff --git a/src/value.rs b/src/value.rs
index dd57a03..2d70084 100644
--- a/src/value.rs
+++ b/src/value.rs
@@ -310,6 +310,18 @@ impl From<&'static str> for NomoValue {
     }
 }
 
+impl From<u64> for NomoValue {
+    fn from(val: u64) -> Self {
+        NomoValue::Integer { value: val }
+    }
+}
+
+impl From<i64> for NomoValue {
+    fn from(val: i64) -> Self {
+        NomoValue::SignedInteger { value: val }
+    }
+}
+
 impl<V> From<Vec<V>> for NomoValue
 where
     V: Into<NomoValue>,
@@ -355,11 +367,43 @@ where
     }
 }
 
-#[cfg(feature = "serde_json")]
 #[derive(Debug, Error, Display)]
-/// Could not transform value to [`NomoValue`]
+/// Could not transform value to/from [`NomoValue`]
 pub struct NomoValueError;
 
+impl TryFrom<NomoValue> for String {
+    type Error = NomoValueError;
+
+    fn try_from(value: NomoValue) -> Result<Self, Self::Error> {
+        match value {
+            NomoValue::String { value } => Ok(value.to_string()),
+            _ => Err(NomoValueError),
+        }
+    }
+}
+
+impl TryFrom<NomoValue> for i64 {
+    type Error = NomoValueError;
+
+    fn try_from(value: NomoValue) -> Result<Self, Self::Error> {
+        match value {
+            NomoValue::SignedInteger { value } => Ok(value),
+            _ => Err(NomoValueError),
+        }
+    }
+}
+
+impl TryFrom<NomoValue> for u64 {
+    type Error = NomoValueError;
+
+    fn try_from(value: NomoValue) -> Result<Self, Self::Error> {
+        match value {
+            NomoValue::Integer { value } => Ok(value),
+            _ => Err(NomoValueError),
+        }
+    }
+}
+
 #[cfg(feature = "serde_json")]
 impl TryFrom<serde_json::Value> for NomoValue {
     type Error = NomoValueError;
diff --git a/tests/file_tests.rs b/tests/file_tests.rs
index db3c666..28e88e9 100644
--- a/tests/file_tests.rs
+++ b/tests/file_tests.rs
@@ -2,6 +2,7 @@ use std::collections::HashMap;
 use std::path::Path;
 
 use nomo::Context;
+use nomo::functions::FunctionMap;
 
 test_each_file::test_each_path! { for ["nomo"] in "./tests/cases/" as cases => check_for_input }
 
@@ -58,7 +59,7 @@ fn check_for_input([path]: [&Path; 1]) {
 
     insta::assert_debug_snapshot!(format!("{basename}.3-instructions"), emit);
 
-    let output = nomo::eval::execute(&emit, &context).unwrap();
+    let output = nomo::eval::execute(&FunctionMap::default(), &emit, &context).unwrap();
 
     insta::assert_debug_snapshot!(format!("{basename}.4-output"), output);
 }