The article describes and discusses what roles customers play in M&A integration. Based on studies of eight domestic and international M&As, it is concluded that customers may: (1) limit integration intentions; (2) be reasons for pre-integration reconsiderations; (3) be used as an argument against integration; (4) not act according to integration intentions; and (5) actively work against integration. Customers’ actual activities, as well as how the M&A parties believe and argue that customers will act, impact the integration. The article highlights how integration is an embedded activity where actions, assumptions and argumentation impact integration. The findings contribute to research on M&As by pointing to how customers impact decisions and outcomes in M&A integration, and by describing integration as an iterative process with several impacting parties.