Naturally, 64-bit versions of Clozure CL require 64-bit processors, for example, a G5 or Core 2. Some early Intel-based Macintoshes used processors that don't support 64-bit operation, so the 64-bit Clozure CL will not run on them, although the 32-bit Clozure CL will.

The 32-bit x86 versions of Clozure CL depend on the presence of the SSE2 instructions. Most x86 processors manufactured and sold in the last several years support SSE2 (all Apple Intel-based Macs do, for instance), but there are some exceptions. The Wikipedia article on SSE2 lists processor models that support SSE2 (and also mentions some of the more notable exceptions).

Clozure CL requires version 2.2.13 (or later) of the Linux kernel and version 2.1.3 (or later) of the GNU C library (glibc) at a bare minimum.

Because of the nature of Linux distributions, it's difficult to give precise version number requirements. In general, a "fairly modern" (no more than 2 or three years old) kernel and C library are more likely to work well than older versions.

The Linux ARM port is relatively new and is still a work-in-progress. Clozure CL needs some features (such as hardware floating-point, locking and memory-serialization primitives) that are only found in chips that implement architecture version 7 (ARMv7); technically, it needs the ARMv7 "application profile", which is sometimes called ARMv7a. In practice, most ARM consumer devices released in the last few years implement ARMv7, but there are exceptions, and it is not practical to enumerate all of the ARM devices that CCL should run on.

In addition to hardware issues, Clozure CL expects Linux to run in little-endian mode and expects software to follow "soft float" calling conventions. The latter has to do with how C functions accept floating-point arguments and return floating-point values.

Clozure CL should run on FreeBSD 6.x and 7.x. FreeBSD 7 users will need to install the "compat6x" package in order to use the distributed Clozure CL kernel, which is built on a FreeBSD 6.x system.

Clozure CL 1.7 runs on Mac OS X (x86) versions 10.5 and later, including 10.7 (Lion),

Clozure CL 1.6 runs on Mac OS X PPC as well as x86 processors.

For Clozure CL versions prior to 1.7, the 32-bit executable does not run under 64-bit Windows.