The purpose of this paper is to describe what roles customers play in merger and acquisition (M&A) integration. Based on studies of eight M&As, it is concluded that customers may (i) limit integration intentions, (ii) be reasons for pre-integration reconsiderations, (iii) be used as an argument against integration, (iv) not act according to integration intentions, and (v) actively work against integration. Customers' actual activities, but also how the M&A parties believe that customers will act, impacts the degree of integration. The paper contributes to research on M&A integration through pointing at how customers impact integration realisation. The paper further contributes to research on M&As according to the network approach through highlighting how an M&A is not merely a response or trigger to change, but an embedded event where actions, beliefs and mutuality impact integration realisation.