We review the concept of endophenotype, and its relationship with related terms in use in the scientific literature, including "biomarker" and "intermediate phenotype". We then explain the importance of the concept in current psychiatry research. Specifically, we focus in the potential importance of endophenotypes in both disentangling the genetic causes, and improving the dimensional and pathophysiologic definition, of psychiatric nosological categories. After delineating which types of variables could constitute valid endophenotypes, we describe efforts in defining neurobiological signatures of social cognitive deficits in schizophrenia as an example with potential heuristic value.