In this initial entry of the Affective Societies: Key Concepts volume we outline a basic understanding of affect circumscribing a general tendency that we deem fruitful as an analytical perspective. This understanding builds on a notion of affect as relational dynamics between evolving bodies in a setting, thus contrasting with approaches to affect as inner states, feelings or emotions. “Affect” designates specifically those encounters between bodies that involve a change – either enhancement or diminishment – in their respective bodily capacities or micro-powers. Thus, affect is inextricable from an approach to power, understood as relations of reciprocal efficaciousness between bodies – human as well as non human – in a particular domain. This suggests an affect based perspective on the dynamic formation and subsequent transformation of individual entities – their ontogenesis and individuation – instead of assuming that entities, whether ordinary objects or human actors, are ready-made, stable and fixed. For human actors, affects are material and ideational relations that, in the short term, increase or diminish their agentive and existential capacities in relation to their surroundings and all other actors and entities present in a situation. In the longer term, affective relations constitute human and nonhuman actors, insofar as affective relations over time both establish and subsequently modulate – make, unmake, remake – individual capacities and dispositions. In other words, relational affect is a central factor in the process of subject formation. Moreover, relational affect is a driving force in the formation and subsequent consolidation of larger aggregates of bodies, that is, in processes of collectivization.