This paper establishes necessary and sufficient conditions for reliable transmission of a source over a relay channel when source side information is available non-causally (a) only at the receiver, (b) only at the relay, or (c) at both the relay and the receiver. For the cases of side information only at the receiver and at both the relay and the receiver, we establish tight necessary and sufficient conditions that apply to any relay channel and show that source-channel separation is optimal. When side information is available only at the relay, we establish a necessary condition for reliable transmission and show that it is tight for the class of degraded relay channels and that source-channel separation is optimal in this case.