QuakeC is a compiled language developed in 1996 by John Carmack of id Software to program parts of the video game Quake. Using QuakeC, a programmer is able to customize Quake to great extents by adding weapons, changing game logic and physics, and programming complex scenarios. It can be used to control many aspects of the game itself, such as parts of the AI, triggers, or changes in the level. The Quake engine was the only game engine to use QuakeC. Following engines used DLL game modules for customization written in C, and C++ from id Tech 4 on.

The QuakeC source to the original id Software Quake game logic was published in 1996 and used as the basis for modifications like capture the flag and others. QuakeC source code is compiled using a tool called qcc into a bytecode kept in a file called progs.dat. The programmers of Quake modifications could then publish their progs.dat bytecode without revealing their source code. Most Quake mods were published this way.

QuakeC is known as interpreted because as Quake runs, it is continually interpreting the progs.dat file.

The syntax of QuakeC is based on that of the C programming language, explaining its name, but it does not support the implementation of new types, structures, arrays, or any kind of referencing other than the "entity" type (which is always a reference). QuakeC also suffers from the fact that many built-in functions (functions prototyped in the QuakeC code but actually defined within the game engine and written in C) return strings in a temporary string buffer, which can only hold one string at any given time. In other words, a construct such as

will fail because the second call to ftos (which converts a floating-point value to a string) overwrites the string returned by the first call before SomeFunction can do something with it. QuakeC does not contain any string handling functions or file handling functions, which were simply not needed by the original game.

Most video games at the time had their game logic written in plain C/C++ and compiled into the executable, which is faster. However, this makes it harder for the community to create mods and it makes the process of porting the game to another platform (such as Linux) more costly.

Despite its advantages, the choice of implementing game logic using a custom scripting language and interpreter was dropped from the next generation Quake II engine in favor of compiled C code due to the overall inflexibility of QuakeC, the increasingly complex game logic, the performance to be gained by packaging game logic into a native dynamic link library, and the advantage of leveraging an already established programming language's community, tools, educational materials, and documentation.

Distributing native code created new security and portability concerns. QuakeC bytecode afforded little opportunity for mischief, while native code has access to the whole machine. QuakeC bytecode also worked on any machine that could run Quake. Compiling to native code added an additional barrier to entry for novice mod developers, because they were being asked to set up a more complicated programming environment. The eventual solution, implemented by the Quake III engine, was to combine the advantages of original QuakeC with the advantages of compiling C to native code. LCC was extended to compile standard C into bytecode, which could be interpreted by a virtual machine in a manner similar to QuakeC. This addressed the security, portability, and tool chain problems, but lost the performance advantage of native code. That was solved by further compiling the bytecode into native code at run time on supported machines.