© 2019 Elsevier Ltd All rights reserved. A signal is considered to be reliable if (1) some feature of the signal is consistently correlated with an attribute of the signaler or its environment and (2) receivers benefit from knowing about that attribute. Signaling systems that do not provide reliable information may exist if signal features exploit a sensory bias of the receiver. At evolutionary equilibrium, however, signals are expected to be reliable on average, meaning that they are reliable enough that a receiver benefits overall from responding to them rather than ignoring them. When there is a conflict of interest between signaler and receiver, signal reliability requires some mechanism to be maintained. Proposed mechanisms include (1) physical and informational constraints on signal production; (2) signal costs that differentially affect signalers of varying quality (the handicap mechanism); (3) differential benefits, which produce honest signals of need; and (4) receiver-dependent costs, which produce conventional signals whose meaning is not related to their physical structure but rather results from an arbitrary convention. The requirement that signals be reliable only on average allows the possibility of some admixture of intra-specific deception, which has been observed in various types of signals, including signals used in aggression, courtship, predator warning, and begging.