The burgeoning field of social signal processing (SSP) is in great need of a theoretical framework to guide future research activities. The present article aims at bringing forward two areas of research that devoted considerable efforts to the understanding of social behaviour: ethology and social psychology. With a long tradition in the study of animal signals, ethology and evolutionary biology have developed theoretical concepts to account for the functional significance of signalling. For example, the consideration of divergent selective pressures responsible for the evolution of signalling and social cognition emphasized the importance of two classes of indicators: informative cues and communicative signals. Social psychology, on the other hand, reports investigations about emotional expression and interpersonal relationships, with a focus on the mechanisms underlying the production and interpretation of social signals and cues. All in all, the concepts developed in these two fields of research should bring invaluable insight to the development of SSP