diff --git a/ctl/conditional.h b/ctl/conditional.h
index 976143a1d..5b63eaa85 100644
--- a/ctl/conditional.h
+++ b/ctl/conditional.h
@@ -17,6 +17,9 @@ struct conditional<false, T, F>
     typedef F type;
 };
 
+template<bool B, typename T, typename F>
+using conditional_t = typename conditional<B, T, F>::type;
+
 } // namespace ctl
 
 #endif // CTL_CONDITIONAL_H_
diff --git a/ctl/is_void.h b/ctl/is_void.h
index 04c33145c..275848d81 100644
--- a/ctl/is_void.h
+++ b/ctl/is_void.h
@@ -19,6 +19,9 @@ template<typename _Tp>
 struct is_void : public is_void_<typename ctl::remove_cv<_Tp>::type>::type
 {};
 
+template<typename T>
+inline constexpr bool is_void_v = is_void<T>::value;
+
 } // namespace ctl
 
 #endif // CTL_IS_VOID_H_
diff --git a/ctl/shared_ptr.h b/ctl/shared_ptr.h
new file mode 100644
index 000000000..2d4ff04f8
--- /dev/null
+++ b/ctl/shared_ptr.h
@@ -0,0 +1,391 @@
+// -*-mode:c++;indent-tabs-mode:nil;c-basic-offset:4;tab-width:8;coding:utf-8-*-
+// vi: set et ft=cpp ts=4 sts=4 sw=4 fenc=utf-8 :vi
+#ifndef CTL_SHARED_PTR_H_
+#define CTL_SHARED_PTR_H_
+
+#include "conditional.h"
+#include "is_convertible.h"
+#include "is_void.h"
+#include "remove_extent.h"
+#include "unique_ptr.h"
+
+namespace ctl {
+
+namespace __ {
+
+static inline __attribute__((always_inline)) void
+incref(size_t* r) noexcept
+{
+#ifdef NDEBUG
+    __atomic_fetch_add(r, 1, __ATOMIC_RELAXED);
+#else
+    size_t refs = __atomic_fetch_add(r, 1, __ATOMIC_RELAXED);
+    if (refs > ((size_t)-1) >> 1)
+        __builtin_trap();
+#endif
+}
+
+static inline __attribute__((always_inline)) bool
+decref(size_t* r) noexcept
+{
+    if (!__atomic_fetch_sub(r, 1, __ATOMIC_RELEASE)) {
+        __atomic_thread_fence(__ATOMIC_ACQUIRE);
+        return true;
+    }
+    return false;
+}
+
+class shared_ref
+{
+  public:
+    constexpr shared_ref() noexcept = default;
+    shared_ref(const shared_ref&) = delete;
+    shared_ref& operator=(const shared_ref&) = delete;
+
+    virtual ~shared_ref() = default;
+
+    void keep_shared() noexcept
+    {
+        incref(&shared);
+    }
+
+    void drop_shared()
+    {
+        if (decref(&shared)) {
+            dispose();
+            drop_weak();
+        }
+    }
+
+    void keep_weak() noexcept
+    {
+        incref(&weak);
+    }
+
+    void drop_weak() noexcept
+    {
+        if (decref(&weak)) {
+            delete this;
+        }
+    }
+
+    size_t use_count() const noexcept
+    {
+        return shared + 1;
+    }
+
+    size_t weak_count() const noexcept
+    {
+        return weak;
+    }
+
+  private:
+    virtual void dispose() = 0;
+
+    size_t shared = 0;
+    size_t weak = 0;
+};
+
+template<typename T, typename D>
+class shared_pointer : public shared_ref
+{
+  public:
+    static shared_pointer* make(T* const p, auto&& d)
+    {
+        auto p2 = unique_ptr(p);
+        return new shared_pointer(p2.release(), forward<decltype(d)>(d));
+    }
+
+  private:
+    shared_pointer(T* const p, auto&& d) noexcept
+      : p(p), d(forward<decltype(d)>(d))
+    {
+    }
+
+    void dispose() override
+    {
+        move(d)(p);
+    }
+
+    T* const p;
+    [[no_unique_address]] D d;
+};
+
+template<typename T, typename U>
+concept shared_ptr_convertible = is_convertible_v<U, T> || is_void_v<T>;
+
+} // namespace __
+
+template<typename T>
+class weak_ptr;
+
+template<typename T>
+class shared_ptr
+{
+  public:
+    using element_type = remove_extent_t<T>;
+    using weak_type = weak_ptr<T>;
+
+    constexpr shared_ptr() noexcept = default;
+    constexpr shared_ptr(nullptr_t) noexcept
+    {
+    }
+
+    template<typename U>
+        requires is_convertible_v<U, T>
+    explicit shared_ptr(U* const p) : shared_ptr(p, default_delete<U>())
+    {
+    }
+
+    template<typename U, typename D>
+        requires is_convertible_v<U, T>
+    shared_ptr(U* const p, D d)
+      : p(p), rc(__::shared_pointer<T, D>::make(p, move(d)))
+    {
+    }
+
+    template<typename U>
+    shared_ptr(const shared_ptr<U>& r, element_type* p) noexcept
+      : p(p), rc(r.rc)
+    {
+        if (rc)
+            rc->keep_shared();
+    }
+
+    template<typename U>
+    shared_ptr(shared_ptr<U>&& r, element_type* p) noexcept : p(p), rc(r.rc)
+    {
+        r.p = nullptr;
+        r.rc = nullptr;
+    }
+
+    template<typename U>
+        requires __::shared_ptr_convertible<T, U>
+    shared_ptr(const shared_ptr<U>& r) noexcept : p(r.p), rc(r.rc)
+    {
+        if (rc)
+            rc->keep_shared();
+    }
+
+    template<typename U>
+        requires __::shared_ptr_convertible<T, U>
+    shared_ptr(shared_ptr<U>&& r) noexcept : p(r.p), rc(r.rc)
+    {
+        r.p = nullptr;
+        r.rc = nullptr;
+    }
+
+    shared_ptr(const shared_ptr& r) noexcept : p(r.p), rc(r.rc)
+    {
+        if (rc)
+            rc->keep_shared();
+    }
+
+    shared_ptr(shared_ptr&& r) noexcept : p(r.p), rc(r.rc)
+    {
+        r.p = nullptr;
+        r.rc = nullptr;
+    }
+
+    template<typename U>
+        requires is_convertible_v<U, T>
+    explicit shared_ptr(const weak_ptr<U>& r) : shared_ptr(r.lock())
+    {
+    }
+
+    template<typename U, typename D>
+        requires is_convertible_v<U, T>
+    shared_ptr(unique_ptr<U, D>&& r)
+      : p(r.p), rc(__::shared_pointer<U, D>::make(r.release(), r.get_deleter()))
+    {
+    }
+
+    ~shared_ptr()
+    {
+        if (rc)
+            rc->drop_shared();
+    }
+
+    shared_ptr& operator=(shared_ptr r) noexcept
+    {
+        swap(r);
+        return *this;
+    }
+
+    template<typename U>
+        requires __::shared_ptr_convertible<T, U>
+    shared_ptr& operator=(shared_ptr<U> r) noexcept
+    {
+        shared_ptr<T>(move(r)).swap(*this);
+        return *this;
+    }
+
+    void reset() noexcept
+    {
+        if (rc) {
+            rc->drop_shared();
+        }
+        p = nullptr;
+        rc = nullptr;
+    }
+
+    template<typename U>
+        requires is_convertible_v<U, T>
+    void reset(U* const p2)
+    {
+        shared_ptr<T>(p2).swap(*this);
+    }
+
+    template<typename U, typename D>
+        requires is_convertible_v<U, T>
+    void reset(U* const p2, D d)
+    {
+        shared_ptr<T>(p2, d).swap(*this);
+    }
+
+    void swap(shared_ptr& r) noexcept
+    {
+        using ctl::swap;
+        swap(p, r.p);
+        swap(rc, r.rc);
+    }
+
+    element_type* get() const noexcept
+    {
+        return p;
+    }
+
+    conditional_t<is_void_v<T>, void, add_lvalue_reference_t<T>> operator*()
+      const noexcept
+    {
+        if (!p)
+            __builtin_trap();
+        return *p;
+    }
+
+    T* operator->() const noexcept
+    {
+        if (!p)
+            __builtin_trap();
+        return p;
+    }
+
+    long use_count() const noexcept
+    {
+        return rc ? rc->use_count() : 0;
+    }
+
+    operator bool() const noexcept
+    {
+        return p;
+    }
+
+    template<typename U>
+    bool owner_before(const shared_ptr<U>& r) const noexcept
+    {
+        return p < r.p;
+    }
+
+    template<typename U>
+    bool owner_before(const weak_ptr<U>& r) const noexcept
+    {
+        return !r.owner_before(*this);
+    }
+
+  private:
+    template<typename U>
+    friend class weak_ptr;
+
+    template<typename U>
+    friend class shared_ptr;
+
+    element_type* p = nullptr;
+    __::shared_ref* rc = nullptr;
+};
+
+template<typename T>
+class weak_ptr
+{
+  public:
+    using element_type = remove_extent_t<T>;
+
+    constexpr weak_ptr() noexcept = default;
+
+    template<typename U>
+        requires is_convertible_v<U, T>
+    weak_ptr(const shared_ptr<U>& r) noexcept : p(r.p), rc(r.rc)
+    {
+        if (rc)
+            rc->keep_weak();
+    }
+
+    ~weak_ptr()
+    {
+        if (rc)
+            rc->drop_weak();
+    }
+
+    long use_count() const noexcept
+    {
+        return rc ? rc->use_count() : 0;
+    }
+
+    bool expired() const noexcept
+    {
+        return !use_count();
+    }
+
+    void reset() noexcept
+    {
+        if (rc)
+            rc->drop_weak();
+        p = nullptr;
+        rc = nullptr;
+    }
+
+    void swap(weak_ptr& r) noexcept
+    {
+        swap(p, r.p);
+        swap(rc, r.rc);
+    }
+
+    shared_ptr<T> lock() const noexcept
+    {
+        if (expired())
+            return nullptr;
+        shared_ptr<T> r;
+        r.p = p;
+        r.rc = rc;
+        if (rc)
+            rc->keep_shared();
+        return r;
+    }
+
+    template<typename U>
+    bool owner_before(const weak_ptr<U>& r) const noexcept
+    {
+        return p < r.p;
+    }
+
+    template<typename U>
+    bool owner_before(const shared_ptr<U>& r) const noexcept
+    {
+        return p < r.p;
+    }
+
+  private:
+    element_type* p = nullptr;
+    __::shared_ref* rc = nullptr;
+};
+
+template<typename T, typename... Args>
+shared_ptr<T>
+make_shared(Args&&... args)
+{
+    // TODO(mrdomino): shared_emplace
+    return shared_ptr<T>(new T(forward<Args>(args)...));
+}
+
+} // namespace ctl
+
+#endif // CTL_SHARED_PTR_H_
diff --git a/test/ctl/shared_ptr_test.cc b/test/ctl/shared_ptr_test.cc
new file mode 100644
index 000000000..6ac7e10ea
--- /dev/null
+++ b/test/ctl/shared_ptr_test.cc
@@ -0,0 +1,221 @@
+// -*- mode:c++; indent-tabs-mode:nil; c-basic-offset:4; coding:utf-8 -*-
+// vi: set et ft=cpp ts=4 sts=4 sw=4 fenc=utf-8 :vi
+//
+// Copyright 2024 Justine Alexandra Roberts Tunney
+//
+// Permission to use, copy, modify, and/or distribute this software for
+// any purpose with or without fee is hereby granted, provided that the
+// above copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+// WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
+// AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
+// DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
+// PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+// TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+#include "ctl/shared_ptr.h"
+#include "ctl/vector.h"
+#include "libc/mem/leaks.h"
+
+// #include <memory>
+// #define ctl std
+
+template<typename T>
+using shared_ptr = ctl::shared_ptr<T>;
+
+template<typename T>
+using weak_ptr = ctl::weak_ptr<T>;
+
+template<typename T, typename... Args>
+shared_ptr<T>
+make_shared(Args&&... args)
+{
+    return ctl::make_shared<T, Args...>(ctl::forward<Args>(args)...);
+}
+
+#undef ctl
+
+static int g = 0;
+
+struct ConstructG
+{
+    ConstructG()
+    {
+        ++g;
+    }
+};
+
+struct DestructG
+{
+    ~DestructG()
+    {
+        ++g;
+    }
+};
+
+struct CallG
+{
+    void operator()(auto*) const noexcept
+    {
+        ++g;
+    }
+};
+
+struct Base
+{};
+
+struct Derived : Base
+{};
+
+int
+main()
+{
+    int a, b;
+
+    {
+        // Shouldn't cause memory leaks.
+        shared_ptr<int> x(new int(5));
+    }
+
+    {
+        // Objects get destroyed when the last shared_ptr is reset.
+        shared_ptr<int> x(&a, CallG());
+        shared_ptr<int> y(x);
+        x.reset();
+        if (g)
+            return 1;
+        y.reset();
+        if (g != 1)
+            return 2;
+    }
+
+    {
+        g = 0;
+        // Weak pointers don't prevent object destruction.
+        shared_ptr<int> x(&a, CallG());
+        weak_ptr<int> y(x);
+        x.reset();
+        if (g != 1)
+            return 3;
+    }
+
+    {
+        g = 0;
+        // Weak pointers can be promoted to shared pointers.
+        shared_ptr<int> x(&a, CallG());
+        weak_ptr<int> y(x);
+        auto z = y.lock();
+        x.reset();
+        if (g)
+            return 4;
+        y.reset();
+        if (g)
+            return 5;
+        z.reset();
+        if (g != 1)
+            return 6;
+    }
+
+    {
+        // Shared null pointers are falsey.
+        shared_ptr<int> x;
+        if (x)
+            return 7;
+        x.reset(new int);
+        if (!x)
+            return 8;
+    }
+
+    {
+        // You can cast a shared pointer validly.
+        shared_ptr<Derived> x(new Derived);
+        shared_ptr<Base> y(x);
+        // But not invalidly:
+        // shared_ptr<Base> x(new Derived);
+        // shared_ptr<Derived> y(x);
+    }
+
+    {
+        // You can cast a shared pointer to void to retain a reference.
+        shared_ptr<int> x(new int);
+        shared_ptr<void> y(x);
+    }
+
+    {
+        // You can take a shared pointer to a subobject, and it will free the
+        // base object.
+        shared_ptr<ctl::vector<int>> x(new ctl::vector<int>);
+        x->push_back(5);
+        shared_ptr<int> y(x, &x->at(0));
+        x.reset();
+        if (*y != 5)
+            return 9;
+    }
+
+    {
+        g = 0;
+        // You can create a shared_ptr from a unique_ptr.
+        ctl::unique_ptr x(&a, CallG());
+        shared_ptr<int> y(ctl::move(x));
+        if (x)
+            return 10;
+        y.reset();
+        if (g != 1)
+            return 11;
+    }
+
+    {
+        g = 0;
+        // You can reassign shared_ptrs.
+        shared_ptr<int> x(&a, CallG());
+        shared_ptr<int> y;
+        y = x;
+        x.reset();
+        if (g)
+            return 12;
+        y.reset();
+        if (g != 1)
+            return 13;
+    }
+
+    {
+        // owner_before works across shared and weak pointers.
+        shared_ptr<int> x(&a, CallG());
+        shared_ptr<int> y(&b, CallG());
+        if (!x.owner_before(y))
+            return 14;
+        if (!x.owner_before(weak_ptr<int>(y)))
+            return 15;
+    }
+
+    {
+        // Use counts work like you'd expect
+        shared_ptr<int> x(new int);
+        if (x.use_count() != 1)
+            return 16;
+        shared_ptr<int> y(x);
+        if (x.use_count() != 2 || y.use_count() != 2)
+            return 17;
+        x.reset();
+        if (x.use_count() != 0 || y.use_count() != 1)
+            return 18;
+    }
+
+    {
+        // There is a make_shared that will allocate an object for you safely.
+        auto x = make_shared<int>(5);
+        if (!x)
+            return 19;
+        if (*x != 5)
+            return 20;
+    }
+
+    // TODO(mrdomino): exercise threads / races. The reference count should be
+    // atomically maintained.
+
+    CheckForMemoryLeaks();
+    return 0;
+}