Channelflow is a software system for numerical analysis of the incompressible Navier-Stokes flow in channel geometries, written in C++. The core engine of Channelflow is a spectral CFD1) algorithm for integrating the Navier-Stokes equations. This engine drives a number of higher-level algorithms that (for example) compute equilibria, traveling waves, and periodic orbits of Navier-Stokes. Channelflow provides these algorithms in an easy-to-use, flexible, and intelligible form by using relatively modern software design. Channelflow consists of a software library for rapid, high-level development of spectral CFD codes and a set of predefined executable programs that perform common tasks involving CFD. Channelflow is customized for Fourier x Chebyshev x Fourier expansions appropriate for rectangular geometries with periodic boundary conditions in two directions and rigid walls in the remaining direction.