G4beamline is a single-particle simulation program optimized for the design and evaluation of beam lines. It is based on the Geant4 toolkit [1], and can implement accurate and realistic simulations of particle transport in both EM fields and matter. This makes it particularly well suited for studies of muon collider and neutrino factory design concepts. G4beamline includes a rich repertoire of beamline elements and is intended to be used directly, without C++ programming, by accelerator physicists. The program has been enhanced to handle a large class of beamline and detector systems, and is available on Linux, Windows, and Macintosh platforms.