OpenGL Shading Language (GLSL) is a high-level shading language with a syntax based on the C programming language. It was created by the OpenGL ARB (OpenGL Architecture Review Board) to give developers more direct control of the graphics pipeline without having to use ARB assembly language or hardware-specific languages.

With advances in graphics cards, new features have been added to allow for increased flexibility in the rendering pipeline at the vertex and fragment level. Programmability at this level is achieved with the use of fragment and vertex shaders.

Originally, this functionality was achieved by writing shaders in ARB assembly language – a complex and unintuitive task. The OpenGL ARB created the OpenGL Shading Language to provide a more intuitive method for programming the graphics processing unit while maintaining the open standards advantage that has driven OpenGL throughout its history.

Originally introduced as an extension to OpenGL 1.4, GLSL was formally included into the OpenGL 2.0 core in 2004 by the OpenGL ARB. It was the first major revision to OpenGL since the creation of OpenGL 1.0 in 1992.

Some benefits of using GLSL are:

• Cross-platform compatibility on multiple operating systems, including Linux, macOS and Windows.
• The ability to write shaders that can be used on any hardware vendor's graphics card that supports the OpenGL Shading Language.
• Each hardware vendor includes the GLSL compiler in their driver, thus allowing each vendor to create code optimized for their particular graphics card’s architecture.

GLSL versions have evolved alongside specific versions of the OpenGL API. It is only with OpenGL versions 3.3 and above that the GLSL and OpenGL major and minor version numbers match. These versions for GLSL and OpenGL are related in the following table:

OpenGL ES and WebGL use OpenGL ES Shading Language (abbreviated: GLSL ES or ESSL).

The two languages are related but not directly compatible. They can be interconverted through SPIRV-Cross.^[18]

GLSL shaders are not stand-alone applications; they require an application that utilizes the OpenGL API, which is available on many different platforms (e.g., Linux, macOS, Windows). There are language bindings for C, C++, C#, JavaScript, Delphi, Java, and many more.

GLSL shaders themselves are simply a set of strings that are passed to the hardware vendor's driver for compilation from within an application using the OpenGL API's entry points. Shaders can be created on the fly from within an application, or read-in as text files, but must be sent to the driver in the form of a string.

The set of APIs used to compile, link, and pass parameters to GLSL programs are specified in three OpenGL extensions, and became part of core OpenGL as of OpenGL Version 2.0. The API was expanded with geometry shaders in OpenGL 3.2, tessellation shaders in OpenGL 4.0 and compute shaders in OpenGL 4.3. These OpenGL APIs are found in the extensions:

• ARB vertex shader
• ARB fragment shader
• ARB shader objects
• ARB geometry shader 4
• ARB tessellation shader
• ARB compute shader

GLSL shaders can also be used with Vulkan, and are a common way of using shaders in Vulkan. GLSL shaders are precompiled before use, or at runtime, into a binary bytecode format called SPIR-V, usually using offline compiler.

• Standard Portable Intermediate Representation, an intermediate shader language by Khronos Group
• 3D computer graphics
• Khronos Group
• WebGL, an OpenGL-ES dialect for web browsers, which uses GLSL for shaders
• Shadertoy
• LWJGL, a library that includes Java bindings for OpenGL.