Though mainstream sociological theory has been founded within dualisms such as structure/agency, nature/culture, and mind/matter, a thread within sociology dating back to Spencer and Tarde favoured a monist ontology that cut across such dualistic categories. This thread has been reinvigorated by recent developments in social theory, including the new materialisms, posthumanism and affect theories. Here we assess what a monist or 'flat' ontology means for sociological understanding of key concepts such as structures and systems, power and resistance. We examine two monistic sociologies: Bruno Latour's 'sociology of associations' and DeLanda's ontology of assemblages. Understandings of social processes in terms of structures, systems or mechanisms are replaced with a focus upon the micropolitics of events and interactions. Power is a flux of forces or 'affects' fully immanent within events, while resistance is similarly an affective flow in events producing micropolitical effects contrary to power or control.