diff --git a/Cargo.lock b/Cargo.lock
index 0356f2f..a4efb43 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1,6 +1,7 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
+version = 3
+
 [[package]]
 name = "estring"
-version = "0.1.2"
-
+version = "0.2.1"
diff --git a/src/agg.rs b/src/agg.rs
index 2d6b12f..ec7c695 100644
--- a/src/agg.rs
+++ b/src/agg.rs
@@ -1,2 +1,6 @@
 //! This module will contain aggregate functions (Sum, Product, etc)
 //!
+
+mod sum;
+
+pub use sum::*;
diff --git a/src/agg/sum.rs b/src/agg/sum.rs
new file mode 100644
index 0000000..8ca61b4
--- /dev/null
+++ b/src/agg/sum.rs
@@ -0,0 +1,73 @@
+use std::marker::PhantomData;
+
+use crate::{Aggregate, EString, ParseFragment};
+
+#[derive(Debug, PartialEq, Eq)]
+struct Sum<R, T>(T, PhantomData<R>)
+where
+    R: Copy + std::iter::Sum,
+    T: ParseFragment + std::ops::Deref<Target = Vec<R>>;
+
+impl<R, T> Sum<R, T>
+where
+    R: Copy + std::iter::Sum,
+    T: ParseFragment + std::ops::Deref<Target = Vec<R>>,
+{
+    fn new(inner: T) -> Self {
+        Self(inner, PhantomData::default())
+    }
+}
+
+impl<R, T> ParseFragment for Sum<R, T>
+where
+    R: Copy + std::iter::Sum,
+    T: ParseFragment + std::ops::Deref<Target = Vec<R>>,
+{
+    fn parse_frag(es: EString) -> crate::Result<Self> {
+        T::parse_frag(es).map(Self::new)
+    }
+}
+
+impl<R, T> Aggregate for Sum<R, T>
+where
+    R: Copy + std::iter::Sum,
+    T: ParseFragment + std::ops::Deref<Target = Vec<R>>,
+{
+    type Target = R;
+
+    fn agg(&self) -> Self::Target {
+        self.0.iter().copied().sum()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::SepVec;
+
+    use super::*;
+
+    #[test]
+    fn should_parse_vec() {
+        let es = EString::from("1,2,3");
+        match es.parse::<Sum<i32, SepVec<i32, ','>>>() {
+            Ok(res) => assert_eq!(res, Sum::new(SepVec::from(vec![1, 2, 3]))),
+            _ => unreachable!(),
+        }
+    }
+
+    #[test]
+    fn should_aggregate_vector() {
+        let es = EString::from("1,2,3");
+        let expr = es.parse::<Sum<i32, SepVec<i32, ','>>>().unwrap();
+        assert_eq!(expr.agg(), 6);
+    }
+
+    #[test]
+    fn should_aggregate_vector_with_inner_aggregation() {
+        let es = EString::from("1+2,2,3");
+        let expr = es
+            .parse::<Sum<_, SepVec<Sum<_, SepVec<i32, '+'>>, ','>>>()
+            .unwrap();
+        assert_eq!(expr.agg(), 6);
+    }
+}
diff --git a/src/core.rs b/src/core.rs
index d47fa93..7fbdef0 100644
--- a/src/core.rs
+++ b/src/core.rs
@@ -110,6 +110,19 @@ pub trait ParseFragment: Sized {
     fn parse_frag(es: EString) -> crate::Result<Self>;
 }
 
+// TODO: add example
+/// Trait to represent structures that can act as aggregators.
+///
+/// The `agg` method works with data that has already been parsed. For this reason, **this trait
+/// should never fail**.
+pub trait Aggregate {
+    /// The resulting type after aggregation.
+    type Target: ?Sized;
+
+    /// Aggregates the value.
+    fn agg(&self) -> Self::Target;
+}
+
 /// Wrapper under ``String`` type.
 ///
 /// # Examples