A discussion is given of waveguides in photonic crystals and branching network geometries of waveguides formed by joining several waveguide channels into conducting circuits for the transmission of light in photonic crystals. We shall refer to these structures in general as photonic crystal circuits. These conducting networks, which transport light, are an optical analogy to electrical circuits, which transport electrons through electrical networks. Photonic crystal circuits, however, unlike most electrical circuits, exhibit a variety of interference effects in their transport properties. The interference effects are related to the nondiffusive nature of the optical transport. The transport properties of light in a variety of circuit geometries are studied. Emphasis is placed on network geometries, which include barriers formed by the addition of dielectric materials to waveguide channels, bends in waveguide channels, closed loops, and interconnecting branched networks. Results for the transmission and reflection properties of photonic circuit modes are presented as functions of the mode frequencies and the dielectric constants of the materials forming the waveguide channels. A comparison is made of the properties of photonic crystal circuits with those of layered optical systems.