diff --git a/src/lib.rs b/src/lib.rs
index 977954f..ee61b18 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,14 +1,225 @@
-use std::{collections::HashMap, hash::RandomState};
+use std::{
+    borrow::Borrow,
+    collections::{hash_map, HashMap, TryReserveError},
+    hash::{BuildHasher, Hash, RandomState},
+    iter::{once, Chain, FlatMap, Flatten, Once},
+};
 
+#[derive(Clone, Debug, Default)]
 pub struct HashMultiSet<T, S = RandomState> {
     data: HashMap<T, Vec<T>, S>,
+    len: usize,
 }
 
+impl<T> HashMultiSet<T, RandomState> {
+    pub fn new() -> Self {
+        Self {
+            data: HashMap::new(),
+            len: 0,
+        }
+    }
+
+    pub fn with_capacity(capacity: usize) -> Self {
+        Self {
+            data: HashMap::with_capacity(capacity),
+            len: 0,
+        }
+    }
+}
+
+impl<T, S> HashMultiSet<T, S> {
+    pub fn capacity(&self) -> usize {
+        self.data.capacity()
+    }
+
+    pub fn iter(&self) -> Iter<'_, T> {
+        self.data.keys().chain(self.data.values().flatten())
+    }
+
+    pub fn len(&self) -> usize {
+        self.len
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.data.is_empty()
+    }
+
+    // TODO: Implement `drain` method
+
+    pub fn retain<F>(&mut self, mut f: F)
+    where
+        F: FnMut(&T) -> bool,
+    {
+        self.data.retain(|k, _| f(k));
+    }
+
+    pub fn clear(&mut self) {
+        self.data.clear();
+        self.len = 0;
+    }
+
+    pub fn with_hasher(hash_builder: S) -> Self {
+        Self {
+            data: HashMap::with_hasher(hash_builder),
+            len: 0,
+        }
+    }
+
+    pub fn with_capacity_and_hasher(capacity: usize, hash_builder: S) -> Self {
+        Self {
+            data: HashMap::with_capacity_and_hasher(capacity, hash_builder),
+            len: 0,
+        }
+    }
+
+    pub fn hasher(&self) -> &S {
+        self.data.hasher()
+    }
+}
+
+impl<T, S> HashMultiSet<T, S>
+where
+    T: Eq + Hash,
+    S: BuildHasher,
+{
+    pub fn reserve(&mut self, additional: usize) {
+        self.data.reserve(additional);
+    }
+
+    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
+        self.data.try_reserve(additional)
+    }
+
+    pub fn shrink_to_fit(&mut self) {
+        self.data.shrink_to_fit();
+    }
+
+    pub fn shrink_to(&mut self, min_capacity: usize) {
+        self.data.shrink_to(min_capacity);
+    }
+
+    // TODO: Implement `diffrence` method
+    // TODO: Implement `symmetric_difference` method
+    // TODO: Implement `intersection` method
+
+    pub fn contains<Q>(&self, value: &Q) -> bool
+    where
+        T: Borrow<Q>,
+        Q: Hash + Eq + ?Sized,
+    {
+        self.data.contains_key(value)
+    }
+
+    pub fn is_disjoint(&self, other: &Self) -> bool {
+        if self.len() <= other.len() {
+            self.data.keys().all(|v| !other.contains(v))
+        } else {
+            other.data.keys().all(|v| !self.contains(v))
+        }
+    }
+
+    pub fn insert(&mut self, value: T) {
+        self.len += 1;
+        #[allow(clippy::map_entry)]
+        if self.data.contains_key(&value) {
+            self.data.get_mut(&value).unwrap().push(value);
+        } else {
+            self.data.insert(value, vec![]);
+        }
+    }
+
+    pub fn remove<Q>(&mut self, value: &Q) -> bool
+    where
+        T: Borrow<Q>,
+        Q: Hash + Eq + ?Sized,
+    {
+        if let Some(v) = self.data.get_mut(value) {
+            if v.is_empty() {
+                self.data.remove(value);
+            } else {
+                self.len -= 1;
+            }
+            true
+        } else {
+            false
+        }
+    }
+
+    pub fn take<Q>(&mut self, value: &Q) -> Option<T>
+    where
+        T: Borrow<Q>,
+        Q: Hash + Eq + ?Sized,
+    {
+        if let Some(v) = self.data.get_mut(value) {
+            if let Some(t) = v.pop() {
+                self.len -= 1;
+                if v.is_empty() {
+                    self.data.remove(value);
+                }
+                Some(t)
+            } else {
+                None
+            }
+        } else {
+            None
+        }
+    }
+}
+
+impl<'a, T, S> IntoIterator for &'a HashMultiSet<T, S> {
+    type Item = &'a T;
+    type IntoIter = Iter<'a, T>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+impl<T> IntoIterator for HashMultiSet<T> {
+    type Item = T;
+    type IntoIter = FlatMap<
+        hash_map::IntoIter<T, Vec<T>>,
+        Chain<Once<T>, std::vec::IntoIter<T>>,
+        fn((T, Vec<T>)) -> Chain<Once<T>, std::vec::IntoIter<T>>,
+    >;
+
+    fn into_iter(self) -> Self::IntoIter {
+        fn f<T>((k, v): (T, Vec<T>)) -> Chain<Once<T>, std::vec::IntoIter<T>> {
+            once(k).chain(v)
+        }
+        self.data.into_iter().flat_map(f)
+    }
+}
+
+impl<T, S> PartialEq for HashMultiSet<T, S>
+where
+    T: Eq + Hash,
+    S: BuildHasher,
+{
+    fn eq(&self, other: &Self) -> bool {
+        if self.len() != other.len() {
+            return false;
+        }
+        self.data
+            .iter()
+            .all(|(k, v)| other.data.get(k).map_or(false, |ov| ov.len() == v.len()))
+    }
+}
+
+impl<T, S> Eq for HashMultiSet<T, S>
+where
+    T: Eq + Hash,
+    S: BuildHasher,
+{
+}
+
+pub type Iter<'a, T> =
+    Chain<hash_map::Keys<'a, T, Vec<T>>, Flatten<hash_map::Values<'a, T, Vec<T>>>>;
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
     #[test]
-    fn it_works() {
-    }
+    fn it_works() {}
 }