We survey the theoretical results obtained for wavelength routing in all--optical networks, present some new results and propose several open problems. In all--optical networks the vast bandwidth available is utilized through {\em wavelength division multiplexing}~: a single physical optical link can carry several logical signals, provided that they are transmitted on different wavelengths. The information, once transmitted as light, reaches its destination without being converted to electronic form in between, thus reaching high data transmission rates. We consider both networks with arbitrary topologies and particular networks of practical interest.