Quilc is an advanced optimizing compiler for the quantum instruction language Quil, licensed under the Apache 2.0 license. Quilc comprises two projects. The first, cl-quil, does the heavy lifting of parsing, compiling, and optimizing Quil code. The second, quilc, presents an external interface for using cl-quil, either using the binary quilc application directly, or alternatively by communicating with an RPCQ server. Quil is the quantum instruction language developed at Rigetti Computing. In Quil quantum algorithms are expressed using Quil's standard gates and instructions. One can also use Quil's DEFGATE to define new non-standard gates, and DEFCIRCUIT to build a named circuit that can be referenced elsewhere in Quil code (analogous to a function in most other programming languages).