This chapter develops a Spinoza- and Deleuze-inspired understanding of affect as relational dynamics between bodies evolving in a setting, in contrast to approaches that see affect as inner states, feelings or emotions. “Affect” designates 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 in a particular domain. 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. In the longer term, affective relations constitute human and non-human actors, insofar as these relations both establish and subsequently modulate individual capacities and dispositions. Delineating affect in these terms is productive for devising research perspectives in a number of fields and with different goals and methodological orientations. Likewise, this approach is generative of further working concepts that can illuminate the nexus between affect, power, and subjectivity.