diff --git a/src/include/interval-map/interval-map.hh b/src/include/interval-map/interval-map.hh
index ce1d9dd..ab3fb44 100644
--- a/src/include/interval-map/interval-map.hh
+++ b/src/include/interval-map/interval-map.hh
@@ -2,6 +2,9 @@
 
 #include <map>
 
+// Testing class forward declaration
+class IntervalMapTest;
+
 namespace amby {
 
 template <typename K, typename V> class interval_map {
@@ -9,17 +12,48 @@ public:
     interval_map(V const& init) : init_(init) {}
 
     void assign(K const& begin, K const& end, V const& val) {
-        // TODO: implement
+        if (!(begin < end))
+            return;
+
+        auto it = underlying_.upper_bound(end);
+        auto const end_val = at_upper_bound(it);
+
+        bool insert_begin = !(val == init_);
+
+        while (it != underlying_.begin()) {
+            it = std::prev(it);
+            auto begin_found = it->first < begin;
+            if (begin_found) {
+                insert_begin = !(val == it->second);
+                break;
+            }
+            if (it != underlying_.end())
+                // Account for up-coming `std::prev` with `++`
+                underlying_.erase(it++);
+        }
+
+        if (insert_begin)
+            it = underlying_.insert(it, {begin, val});
+        // Get the proper upper-bound for `end`
+        it = (it == underlying_.end()) ? it : std::next(it);
+        if (!(at_upper_bound(it) == end_val))
+            underlying_.insert(it, {end, end_val});
     }
 
     V const& operator[](K const& key) const {
-        auto it = underlying_.upper_bound(key);
+        return at_upper_bound(underlying_.upper_bound(key));
+    }
+
+    // Used in testing
+    friend class ::IntervalMapTest;
+
+private:
+    V const& at_upper_bound(std::map<K, V>::const_iterator it) const {
         if (it == underlying_.begin())
             return init_;
         return std::prev(it)->second;
     }
 
-private:
     V init_;
     std::map<K, V> underlying_{};
 };
diff --git a/tests/unit/model.hh b/tests/unit/model.hh
new file mode 100644
index 0000000..9acb19f
--- /dev/null
+++ b/tests/unit/model.hh
@@ -0,0 +1,35 @@
+#pragma once
+
+#include <vector>
+
+template <typename K, typename V> struct Range {
+    K begin;
+    K end;
+    V val;
+};
+
+// Same behaviour as interval_map, but implementation is trivally correct
+template <typename K, typename V> class Model {
+public:
+    Model(V const& init) : init_(init) {}
+
+    void assign(K const& begin, K const& end, V const& val) {
+        if (!(begin < end))
+            return;
+        ranges_.emplace_back(begin, end, val);
+    }
+
+    V const& operator[](K const& key) const {
+        for (auto it = ranges_.rbegin(); it != ranges_.rend(); ++it) {
+            if (key < it->begin)
+                continue;
+            if (it->end <= key)
+                continue;
+            return it->val;
+        }
+        return init_;
+    }
+
+    V init_;
+    std::vector<Range<K, V>> ranges_{};
+};
diff --git a/tests/unit/unit_test.cc b/tests/unit/unit_test.cc
index 288c811..c429d29 100644
--- a/tests/unit/unit_test.cc
+++ b/tests/unit/unit_test.cc
@@ -2,8 +2,12 @@
 
 #include <interval-map/interval-map.hh>
 
+#include <random>
+#include <sstream>
 #include <type_traits>
 
+#include "model.hh"
+
 template <typename T> class KeyInterface {
 public:
     explicit KeyInterface(T val) : underlying_(val) {}
@@ -58,7 +62,82 @@ static_assert(std::is_same_v<
               ValueInterface<int>,
               decltype(std::numeric_limits<ValueInterface<int>>::lowest())>);
 
-TEST(interval_map, minimal_interface) {
+class IntervalMapTest : public testing::Test {
+protected:
+    using key_type = char;
+    using value_type = int;
+    using map_type = amby::interval_map<key_type, value_type>;
+    using model_type = Model<key_type, value_type>;
+
+    map_type map{0};
+    model_type model{0};
+
+    void SetUp() override {
+        map = map_type{0};
+        model = model_type{0};
+    }
+
+    void TearDown() override {
+        check();
+    }
+
+    void assign(key_type const& begin, key_type const& end,
+                value_type const& val) {
+        map.assign(begin, end, val);
+        model.assign(begin, end, val);
+    }
+
+    void check() const {
+        SCOPED_TRACE(stringify_map());
+        SCOPED_TRACE(stringify_operations());
+        check_ranges();
+        check_canonicity();
+    }
+
+    std::string stringify_map() const {
+        std::ostringstream out;
+        out << "map: ";
+        for (const auto& [key, val] : map.underlying_)
+            out << "[" << +key << ": " << +val << "]";
+        return out.str();
+    }
+
+    std::string stringify_operations() const {
+        std::ostringstream out;
+        out << "ops: ";
+        for (const auto& [start, end, val] : model.ranges_)
+            out << "[" << +start << ":" << +end << " => " << +val << "]";
+        return out.str();
+    }
+
+    // Compare against the fake 'Model' implementation
+    void check_ranges() const {
+        auto i = std::numeric_limits<key_type>::min();
+        for (; i < std::numeric_limits<key_type>::max(); ++i) {
+            ASSERT_EQ(map[i], model[i]) << "(i: " << +i << ")";
+        }
+        ASSERT_EQ(map[i], model[i]) << "(i: " << +i << ")";
+    };
+
+    void check_canonicity() const {
+        // Consecutive map entries must not contain the same value
+        for (auto it = map.underlying_.begin(); it != map.underlying_.end();
+             ++it) {
+            const auto next = std::next(it, 1);
+            if (next == map.underlying_.end())
+                break;
+            EXPECT_NE(it->second, next->second);
+        }
+
+        // The first entry must not contain the initial value
+        if (const auto it = map.underlying_.begin();
+            it != map.underlying_.end()) {
+            EXPECT_NE(it->second, map.init_);
+        }
+    }
+};
+
+TEST_F(IntervalMapTest, minimal_interface) {
     using Key = KeyInterface<char>;
     using Value = ValueInterface<int>;
 
@@ -66,3 +145,121 @@ TEST(interval_map, minimal_interface) {
     ASSERT_EQ(map[Key(0)], Value(0));
     map.assign(Key(0), Key(1), Value(1));
 }
+
+TEST_F(IntervalMapTest, no_insertion) {}
+
+TEST_F(IntervalMapTest, insert_begin_equal_end) {
+    assign(0, 0, 1);
+}
+
+TEST_F(IntervalMapTest, insert_begin_bigger_than_end) {
+    assign(1, 0, 1);
+}
+
+TEST_F(IntervalMapTest, insert_one_range) {
+    assign(std::numeric_limits<key_type>::min(), 0, 1);
+}
+
+TEST_F(IntervalMapTest, insert_non_overlapping_ranges) {
+    assign(std::numeric_limits<key_type>::min(), 0, 1);
+    assign(10, std::numeric_limits<key_type>::max(), 2);
+}
+
+TEST_F(IntervalMapTest, insert_up_to_max) {
+    assign(std::numeric_limits<key_type>::min(),
+           std::numeric_limits<key_type>::max(), 1);
+}
+
+TEST_F(IntervalMapTest, insert_range_right_after) {
+    assign(0, 10, 1);
+    assign(10, 20, 1);
+}
+
+TEST_F(IntervalMapTest, insert_range_right_before) {
+    assign(10, 20, 1);
+    assign(0, 10, 1);
+}
+
+TEST_F(IntervalMapTest, insert_range_middle) {
+    assign(0, 10, 1);
+    assign(20, 30, 1);
+    assign(10, 20, 1);
+}
+
+TEST_F(IntervalMapTest, insert_range_inside_another) {
+    assign(0, 20, 1);
+    assign(5, 15, 2);
+}
+
+TEST_F(IntervalMapTest, insert_range_around_another) {
+    assign(5, 15, 2);
+    assign(0, 20, 1);
+}
+
+TEST_F(IntervalMapTest, insert_range_overlaps_many) {
+    assign(0, 10, 1);
+    assign(10, 20, 2);
+    assign(20, 30, 3);
+    assign(30, 40, 4);
+    assign(40, 50, 5);
+    assign(0, 50, -1);
+}
+
+TEST_F(IntervalMapTest, insert_range_overlaps_many_init_value) {
+    assign(0, 10, 1);
+    assign(10, 20, 2);
+    assign(20, 30, 3);
+    assign(30, 40, 4);
+    assign(40, 50, 5);
+    assign(0, 50, 0);
+}
+
+TEST_F(IntervalMapTest, insert_range_overlaps_many_oversize) {
+    assign(0, 10, 1);
+    assign(10, 20, 2);
+    assign(20, 30, 3);
+    assign(30, 40, 4);
+    assign(40, 50, 5);
+    assign(-10, 60, -1);
+}
+
+TEST_F(IntervalMapTest, fuzzing_001) {
+    assign(-50, 20, 1);
+    assign(40, 80, 2);
+    assign(-100, -10, 3);
+}
+
+TEST_F(IntervalMapTest, fuzzing_002) {
+    assign(-100, 90, 1);
+    assign(0, 120, 2);
+    assign(-60, 60, 3);
+}
+
+TEST_F(IntervalMapTest, fuzzing_003) {
+    assign(-80, 70, 1);
+    assign(-50, 40, 2);
+    assign(-40, 20, 3);
+    assign(-110, -10, 4);
+}
+
+TEST_F(IntervalMapTest, randomized_test) {
+    auto const seed = []() {
+        std::random_device r;
+        return r();
+    }();
+    SCOPED_TRACE(seed);
+    auto random = std::mt19937_64(seed);
+
+    auto keys = std::uniform_int_distribution<key_type>(
+        std::numeric_limits<key_type>::min(),
+        std::numeric_limits<key_type>::max());
+    auto values = std::uniform_int_distribution<value_type>(0, 10);
+
+    for (auto i = 0; i < 1000; ++i) {
+        auto const start = keys(random);
+        auto const end = keys(random);
+        auto const value = values(random);
+        assign(start, end, value);
+        check();
+    }
+}