Quartz Composer is a node graph system provided as part of the Xcode development environment in macOS for processing and rendering graphical data. It is capable of making sophisticated animations for keynote or presentations and creating animated screensavers.

Quartz Composer uses OpenGL (including GLSL), OpenCL (only in Mac OS X Snow Leopard and later), OpenAL, Core Image, Core Video, JavaScript and other technologies to create an API and a developer tool around a simple visual programming paradigm. Apple has embedded Quartz technologies deeply into the operating system. Compositions created in Quartz Composer can be played standalone in any QuickTime-aware application (although only on Mac OS X Tiger and later), as a system Screen Saver, as an iTunes Visualizer, from inside the Quartz Composer application, or can be embedded into a Cocoa or Carbon application via supplied user interface widgets. While Quartz Composer is included with the iPhone SDK, as of December 2015^[update] there is no way of running Quartz Compositions on iOS devices. Starting in macOS Catalina, the Quartz Composer framework has been deprecated, although it is still present for compatibility.

Quartz programming through Quartz Composer works by implementing and connecting patches. Similar to routines in traditional programming languages, patches are base processing units. They execute and produce a result. For better performance, patch execution follows a lazy evaluation approach, meaning that patches are only executed when their output is needed. There are three types of patches: Consumers, Processors and External Input patches that can receive and output mouse clicks, scrolls and movements; MIDI and audio; keyboard; or other movements. A collection of patches can be melded into one, called a macro. Macros can be nested and their subroutines also edited.

To control the order of rendering, each renderer is assigned a layer, indicated in its upper-right corner. Layers are rendered sequentially, lowest to highest. Renderers can be enabled or disabled, essentially turning on or off that particular layer. Turning off unused layers often results in better performance, since fewer upstream patches need to be evaluated.

Some patches can have subpatches, which allows for global parameter changes to just the included subpatches. This is useful for lighting, 3D transformation and GLSL shaders, among other things. Subpatch support is indicated by square corners on a patch, rather than the typical rounded corners.

With Version 3.0, it became possible to turn compositions into Virtual Patches. These allow the user to reuse functionality without having to store duplicate copies in each composition. The Quartz Composer Editor allows the user to save a "flattened" copy (with the virtual patches fully expanded inside), for easy distribution. Version 4.0 extended this functionality even more and automatically includes "flattened" copies of virtual patches for use as a fallback if the desired virtual patch isn't installed on the host system. This greatly simplifies composition distribution.

Network functionality was greatly improved with the release of Leopard. It became possible to transmit data and synchronize over a network interface and it also added support for Open Sound Control transmission and reception.

Also new in Version 3.0 was the possibility to write custom patch plugins using an Xcode template, and the notion of a "safe mode" where plugins and other unsafe patches fail to load. This prevents malicious compositions from performing dangerous or insecure operations. Custom patches using Apple's Xcode template are always considered unsafe.