||Interpersonal synchrony in social interaction between interactive partners is the dynamic and reciprocal adaptation of their verbal and nonverbal behaviors. It affords both a novel domain for computer vision and machine learning, as well as a novel context with which to examine individual variation in cognitive, physiological, and neural processes in the interacting members. This talk will introduce basic principle of synchrony and its current research challenges. Examples from research carried out in European and national projects on human-human and human-robot interaction will be show and discussed. Finally, the open Python library SyncPy, recently developed at the IMI2S Group at Université Pierre et Marie Curie-Pars VI, for the analysis of synchrony will be presented.