This article introduces the concept of affective resonance, which names a type of affective dynamics of reciprocal modulation between interactants. Resonance arises through a complex interplay between capacities to affect and be affected of multiple things and individuals, and in this interplay, active and receptive affects are inextricably entangled. Affective resonance is thus based on a fundamental reciprocity at the level of the causality of affect, while at the level of the resulting forms and contours, resonance dynamics may be complementary, or even asymmetric, which distinguishes resonance from imitation and synchronization. The concept is presented in the context of a relational and dynamic understanding of affect on the basis of Spinoza and Deleuze. The legacy of the concept in physics and the relation to neighboring concepts in affect studies, philosophy and developmental psychology is briefly discussed.