diff --git a/scripts/grade-12.sh b/scripts/grade-12.sh
new file mode 100755
index 0000000..6a87f98
--- /dev/null
+++ b/scripts/grade-12.sh
@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+
+set -e
+set -uo pipefail
+IFS=$'\n\t'
+
+# Imports library.
+BASEDIR=$(dirname "$0")
+source $BASEDIR/grade-utils.sh
+
+RUNNERS=(
+    "cargo"
+    "cargo --release"
+    "cargo_asan"
+    "cargo_asan --release"
+    "cargo_tsan"
+    "cargo_tsan --release"
+)
+
+# Lints.
+run_linters || exit 1
+
+# Executes test for each runner.
+for RUNNER in "${RUNNERS[@]}"; do
+    echo "Running with $RUNNER..."
+
+    TESTS=("--lib assignment12_grade")
+    if [ $(run_tests) -ne 0 ]; then
+        exit 1
+    fi
+done
+
+exit 0
diff --git a/scripts/grade-13.sh b/scripts/grade-13.sh
new file mode 100755
index 0000000..6c180d4
--- /dev/null
+++ b/scripts/grade-13.sh
@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+
+set -e
+set -uo pipefail
+IFS=$'\n\t'
+
+# Imports library.
+BASEDIR=$(dirname "$0")
+source $BASEDIR/grade-utils.sh
+
+RUNNERS=(
+    "cargo"
+    "cargo --release"
+    "cargo_asan"
+    "cargo_asan --release"
+    "cargo_tsan"
+    "cargo_tsan --release"
+)
+
+# Lints.
+run_linters || exit 1
+
+# Executes test for each runner.
+for RUNNER in "${RUNNERS[@]}"; do
+    echo "Running with $RUNNER..."
+
+    TESTS=("--lib assignment13_grade")
+    if [ $(run_tests) -ne 0 ]; then
+        exit 1
+    fi
+done
+
+exit 0
diff --git a/src/assignments/assignment03.rs b/src/assignments/assignment03.rs
index f30136a..f51b26c 100644
--- a/src/assignments/assignment03.rs
+++ b/src/assignments/assignment03.rs
@@ -50,6 +50,8 @@ pub enum MyOption<T> {
 /// Converts an `MyOption<String>` into an `MyOption<usize>`, consuming the original:
 ///
 /// ```
+/// use cs220::assignments::assignment03::{my_map, MyOption};
+///
 /// fn len(s: String) -> usize {
 ///     s.len()
 /// }
@@ -68,6 +70,8 @@ pub fn my_map<T, U, F: FnOnce(T) -> U>(v: MyOption<T>, f: F) -> MyOption<U> {
 /// # Examples
 ///
 /// ```
+/// use cs220::assignments::assignment03::{MyOption, my_and_then};
+///
 /// fn pos_then_to_string(x: isize) -> MyOption<String> {
 ///     if x > 0 {
 ///         MyOption::MySome(x.to_string())
@@ -93,13 +97,13 @@ pub fn my_and_then<T, U, F: FnOnce(T) -> MyOption<U>>(v: MyOption<T>, f: F) -> M
 ///
 /// For example, the following list of seven numbers,
 ///
-/// ```
+/// ```ignore
 /// vec![1, 3, 3, 6, 7, 8, 9]
 /// ```
 ///
 /// has the median of 6, which is the fourth value. And for this data set of eight numbers,
 ///
-/// ```
+/// ```ignore
 /// vec![1, 2, 3, 4, 5, 6, 8, 9]
 /// ```
 ///
@@ -142,13 +146,13 @@ pub fn piglatin(input: String) -> String {
 ///
 /// If the commands are as follows:
 ///
-/// ```
+/// ```ignore
 /// vec!["Add Amir to Engineering", "Add Sally to Sales", "Remove Jeehoon from Sales", "Move Amir from Engineering to Sales"]
 /// ```
 ///
 /// The return value should be:
 ///
-/// ```
+/// ```ignore
 /// ["Sales" -> ["Amir", "Sally"]]
 /// ```
 ///
diff --git a/src/assignments/assignment06.rs b/src/assignments/assignment06.rs
index 56a8168..e9a9646 100644
--- a/src/assignments/assignment06.rs
+++ b/src/assignments/assignment06.rs
@@ -91,7 +91,7 @@ impl Semiring for f64 {
 ///
 /// For example, polynomial `x^2 + 5x + 6` is represented in `Polynomial<u64>` as follows:
 ///
-/// ```
+/// ```ignore
 /// Polynomial {
 ///     coefficients: {
 ///         2: 1,
diff --git a/src/assignments/assignment09.rs b/src/assignments/assignment09.rs
index 2e31ab7..38dec0e 100644
--- a/src/assignments/assignment09.rs
+++ b/src/assignments/assignment09.rs
@@ -16,6 +16,8 @@ use itertools::*;
 /// # Exmample
 ///
 /// ```
+/// use cs220::assignments::assignment09::is_fibonacci;
+///
 /// assert_eq!(is_fibonacci([1, 1, 2, 3, 5, 8, 13].into_iter()), true);
 /// assert_eq!(is_fibonacci([1, 1, 2, 3, 5, 8, 14].into_iter()), false);
 /// ```
@@ -28,6 +30,8 @@ pub fn is_fibonacci(inner: impl Iterator<Item = i64>) -> bool {
 /// # Exmaple
 ///
 /// ```
+/// use cs220::assignments::assignment09::sigma;
+///
 /// assert_eq!(sigma([1, 2].into_iter(), |x| x + 2), 7);
 /// assert_eq!(sigma([1, 2].into_iter(), |x| x * 4), 12);
 /// ```
@@ -42,6 +46,8 @@ pub fn sigma<T, F: Fn(T) -> i64>(inner: impl Iterator<Item = T>, f: F) -> i64 {
 /// # Example
 ///
 /// ```
+/// use cs220::assignments::assignment09::interleave3;
+///
 /// assert_eq!(
 ///     interleave3([1, 2].into_iter(), [3, 4].into_iter(), [5, 6].into_iter()),
 ///     vec![1, 3, 5, 2, 4, 6]
@@ -60,6 +66,8 @@ pub fn interleave3<T>(
 /// # Example
 ///
 /// ```
+/// use cs220::assignments::assignment09::k_smallest_mean;
+///
 /// assert_eq!(
 ///     k_smallest_mean(vec![1, 3, 2].into_iter(), 2),
 ///     ((1 + 2) as f64 / 2.0)
@@ -78,6 +86,8 @@ pub fn k_smallest_mean(inner: impl Iterator<Item = i64>, k: usize) -> f64 {
 /// # Exmaple
 ///
 /// ```
+/// use cs220::assignments::assignment09::calculate_mean;
+///
 /// assert_eq!(
 ///     calculate_mean(
 ///         [
@@ -111,6 +121,8 @@ pub fn calculate_mean(inner: impl Iterator<Item = (String, i64)>) -> HashMap<Str
 /// Among these sets, the number of sets whose sum is 4 is 2, which is [1, 3] and [2, 2].
 ///
 /// ```
+/// use cs220::assignments::assignment09::sum_is_n;
+///
 /// assert_eq!(sum_is_n(vec![vec![1, 2, 3], vec![2, 3]], 3), 1);
 /// assert_eq!(sum_is_n(vec![vec![1, 2, 3], vec![2, 3]], 4), 2);
 /// assert_eq!(sum_is_n(vec![vec![1, 2, 3], vec![2, 3]], 5), 2);
@@ -126,6 +138,8 @@ pub fn sum_is_n(inner: Vec<Vec<i64>>, n: i64) -> usize {
 /// # Example
 ///
 /// ```
+/// use cs220::assignments::assignment09::find_count_n;
+///
 /// assert_eq!(find_count_n(vec![1, 2], 1), vec![1, 2]);
 /// assert_eq!(find_count_n(vec![1, 3, 3], 1), vec![1]);
 /// assert_eq!(find_count_n(vec![1, 3, 3], 2), vec![3]);
@@ -150,6 +164,8 @@ pub fn find_count_n(inner: Vec<usize>, n: usize) -> Vec<usize> {
 /// # Exmaple
 ///
 /// ```
+/// use cs220::assignments::assignment09::position_median;
+///
 /// assert_eq!(position_median(vec![1, 3, 3, 6, 7, 8, 9]), Some(3));
 /// assert_eq!(position_median(vec![1, 3, 3, 3]), Some(1));
 /// ```
diff --git a/src/assignments/assignment10.rs b/src/assignments/assignment10.rs
index 37c8449..6a84161 100644
--- a/src/assignments/assignment10.rs
+++ b/src/assignments/assignment10.rs
@@ -47,7 +47,7 @@ pub enum Node<T> {
 ///
 /// For example, the result of preorder traversal for the following tree
 ///
-/// ```
+/// ```ignore
 ///     1
 ///    /|\
 ///   2 3 4
@@ -57,7 +57,7 @@ pub enum Node<T> {
 ///
 /// which can be represented as
 ///
-/// ```
+/// ```ignore
 /// Node::NonLeaf((
 ///     1,
 ///     vec![
diff --git a/src/assignments/assignment12.rs b/src/assignments/assignment12.rs
new file mode 100644
index 0000000..ec381ee
--- /dev/null
+++ b/src/assignments/assignment12.rs
@@ -0,0 +1,33 @@
+#![allow(single_use_lifetimes)]
+
+//! Assignment 12: Concurrency.
+//!
+//! The primary goal of this assignment is to get used to concurrency.
+//!
+//! You should fill out the `todo!()` placeholders in such a way that `/scripts/grade-12.sh` works fine.
+//! See `assignment12_grade.rs` and `/scripts/grade-12.sh` for the test script.
+
+use std::sync::mpsc::{Receiver, RecvError, Sender};
+use std::thread;
+
+use etrace::*;
+
+/// The "pong" function (read the test script to figure out what it should do).
+pub fn pong(rx1: &mut Receiver<u32>, tx2: &mut Sender<u32>) -> bool {
+    todo!()
+}
+
+/// Executes the given functions (f1, f2) and returns the results.
+pub fn use_scoped_thread<'scope, 'env, T1, T2, F1, F2>(
+    s: &'scope thread::Scope<'scope, 'env>,
+    f1: F1,
+    f2: F2,
+) -> (T1, T2)
+where
+    T1: Send + 'scope,
+    T2: Send + 'scope,
+    F1: Send + FnOnce() -> T1 + 'scope,
+    F2: Send + FnOnce() -> T2 + 'scope,
+{
+    todo!()
+}
diff --git a/src/assignments/assignment12_grade.rs b/src/assignments/assignment12_grade.rs
new file mode 100644
index 0000000..332fe58
--- /dev/null
+++ b/src/assignments/assignment12_grade.rs
@@ -0,0 +1,44 @@
+#[cfg(test)]
+mod test {
+    use super::super::assignment12::*;
+
+    use std::sync::mpsc::channel;
+    use std::thread;
+
+    #[test]
+    fn test_ping_pong() {
+        let (tx1, mut rx1) = channel();
+        let (mut tx2, rx2) = channel();
+
+        let thread_ping = thread::spawn(move || {
+            for i in 0..100 {
+                tx1.send(i).unwrap();
+                let x = rx2.recv().unwrap();
+                assert_eq!(x, i + 1);
+            }
+        });
+
+        let thread_pong = thread::spawn(move || while pong(&mut rx1, &mut tx2) {});
+
+        thread_ping.join().unwrap();
+        thread_pong.join().unwrap();
+    }
+
+    #[test]
+    fn test_scoped_thread() {
+        for i in 0..100 {
+            let v = (0..i).collect::<Vec<u32>>();
+
+            thread::scope(|s| {
+                let (r1, r2) = use_scoped_thread(
+                    s,
+                    || v.iter().sum::<u32>(),
+                    || v.windows(2).map(|x| x[0] * x[1]).sum::<u32>(),
+                );
+
+                assert_eq!(r1, v.iter().sum());
+                assert_eq!(r2, v.windows(2).map(|x| x[0] * x[1]).sum());
+            });
+        }
+    }
+}
diff --git a/src/assignments/assignment13.rs b/src/assignments/assignment13.rs
new file mode 100644
index 0000000..bd91185
--- /dev/null
+++ b/src/assignments/assignment13.rs
@@ -0,0 +1,47 @@
+//! Assignment 13: Parallelism.
+//!
+//! The primary goal of this assignment is to get used to data parallelism.
+//!
+//! You should fill out the `todo!()` placeholders in such a way that `/scripts/grade-13.sh` works fine.
+//! See `assignment13_grade.rs` and `/scripts/grade-13.sh` for the test script.
+
+use rayon::prelude::*;
+
+/// Returns the sum of `f(v)` for all element `v` the given array.
+///
+/// # Exmaple
+///
+/// ```
+/// use cs220::assignments::assignment13::sigma;
+/// use rayon::iter::IntoParallelIterator;
+///
+/// assert_eq!(sigma([1, 2].into_par_iter(), |x| x + 2), 7);
+/// assert_eq!(sigma([1, 2].into_par_iter(), |x| x * 4), 12);
+/// ```
+pub fn sigma<T, F: Fn(T) -> i64 + Sync + Send>(
+    inner: impl ParallelIterator<Item = T>,
+    f: F,
+) -> i64 {
+    todo!()
+}
+
+/// Alternate elements from three iterators until they have run out.
+///
+/// # Example
+///
+/// ```
+/// use cs220::assignments::assignment13::interleave3;
+/// use rayon::iter::IntoParallelIterator;
+///
+/// assert_eq!(
+///     interleave3([1, 2].into_par_iter(), [3, 4].into_par_iter(), [5, 6].into_par_iter()),
+///     vec![1, 3, 5, 2, 4, 6]
+/// );
+/// ```
+pub fn interleave3<T: Send>(
+    list1: impl IndexedParallelIterator<Item = T>,
+    list2: impl IndexedParallelIterator<Item = T>,
+    list3: impl IndexedParallelIterator<Item = T>,
+) -> Vec<T> {
+    todo!()
+}
diff --git a/src/assignments/assignment13_grade.rs b/src/assignments/assignment13_grade.rs
new file mode 100644
index 0000000..6f560c6
--- /dev/null
+++ b/src/assignments/assignment13_grade.rs
@@ -0,0 +1,67 @@
+#[cfg(test)]
+mod test {
+    use super::super::assignment13::*;
+    use rayon::prelude::IntoParallelIterator;
+
+    #[test]
+    fn test_sigma() {
+        assert_eq!(sigma([].into_par_iter(), |x: i64| x * 2), 0);
+        assert_eq!(sigma([1].into_par_iter(), |x| x * 3), 3);
+        assert_eq!(sigma([1, 2].into_par_iter(), |x| x + 2), 7);
+        assert_eq!(sigma([1, 2].into_par_iter(), |x| x * 4), 12);
+        assert_eq!(sigma([1, 2, 3].into_par_iter(), |x| x * 5), 30);
+
+        assert_eq!(
+            sigma([-1.2, 3.0, 4.2, 5.8].into_par_iter(), |x: f64| x.floor()
+                as i64),
+            10
+        );
+        assert_eq!(
+            sigma([-1.2, 3.0, 4.2, 5.8].into_par_iter(), |x: f64| x.ceil()
+                as i64),
+            13
+        );
+        assert_eq!(
+            sigma([-1.2, 3.0, 4.2, 5.8].into_par_iter(), |x: f64| x.round()
+                as i64),
+            12
+        );
+
+        assert_eq!(
+            sigma(["Hello,", "World!"].into_par_iter(), |x| x.len() as i64),
+            12
+        );
+    }
+
+    #[test]
+    fn test_interleave3() {
+        assert_eq!(
+            interleave3(
+                [1, 2].into_par_iter(),
+                [3, 4].into_par_iter(),
+                [5, 6].into_par_iter()
+            ),
+            vec![1, 3, 5, 2, 4, 6]
+        );
+
+        assert_eq!(
+            interleave3(
+                [1, 2, 3].into_par_iter(),
+                [4, 5, 6].into_par_iter(),
+                [7, 8, 9].into_par_iter()
+            ),
+            vec![1, 4, 7, 2, 5, 8, 3, 6, 9]
+        );
+
+        assert_eq!(
+            interleave3(
+                ["a", "b", "c"].into_par_iter(),
+                ["d", "e", "f"].into_par_iter(),
+                ["g", "h", "i"].into_par_iter()
+            )
+            .into_iter()
+            .collect::<String>(),
+            "adgbehcfi"
+        );
+    }
+}
diff --git a/src/assignments/mod.rs b/src/assignments/mod.rs
index b69cbdb..74e67fe 100644
--- a/src/assignments/mod.rs
+++ b/src/assignments/mod.rs
@@ -26,3 +26,7 @@ pub mod assignment10;
 mod assignment10_grade;
 pub mod assignment11;
 mod assignment11_grade;
+pub mod assignment12;
+mod assignment12_grade;
+pub mod assignment13;
+mod assignment13_grade;