In CPython, the global interpreter lock, or GIL, is a mutex that prevents multiple native threads from running Python code at once. This lock is necessary mainly because CPython's memory management is not thread-safe. (However, since the GIL exists, other features have grown to depend on the guarantees that it enforces.)

CPython extensions must be GIL-aware in order to avoid defeating threads. For an explanation, see [http://docs.python.org/api/threads.html Global interpreter lock].

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One of IronPython's delights is that it works around the GIL; IronPython-based applications can be fully-threaded in the sense that they properly exploit multi-core CPUs.

[Mention place of GIL in StacklessPython.]