This chapter summarizes theoretically achievable gains and the construction of practical codes for user-cooperation. Most of these results relate to the relay channel, which is a three-terminal channel that captures the essence of usercooperation and serves as one of the primary building blocks for cooperation on a larger scale. In investigating the fundamental limits of relaying, we present information-theoretic results on the achievable throughput of relay channel in mutual-information terms. We also include results on Gaussian channels, and for the practically important case of half-duplex relaying. In the domain of relay coding, we specifically discuss pragmatic code constructions for half as well as full-duplex relaying, using LDPC codes as components.