Synchrony refers to individuals' temporal coordination during social interactions. The analysis of this phenomenon is complex, requiring the perception and integration of multimodal communicative signals. The evaluation of synchrony has received multidisciplinary attention because of its role in early development, language learning, and social connection. Originally studied by developmental psychologists, synchrony has now captured the interest of researchers in such fields as social signal processing, robotics, and machine learning. This paper emphasizes the current questions asked by synchrony evaluation and the state-of-the-art related methods. First, we present definitions and functions of synchrony in youth and adulthood. Next, we review the noncomputational and computational approaches of annotating, evaluating, and modeling interactional synchrony. Finally, the current limitations and future research directions in the fields of developmental robotics, social robotics, and clinical studies are discussed.