This paper examines the dynamics of a post-merger and acquisition (M&A) integration process. Integrating a merger or acquisition is a dynamically complex managerial challenge and our combination of fieldwork and simulation modelling provides new insights for scholars, policy makers, and practicing managers. From our field data, we identify four common behaviour modes for the performance of an M&A integration over time and develop a simulation model capable of generating all four behaviour models. Simulation analyses highlight the important role of several feedback processes that have not been identified in prior work: (1) the ‘Achieving target synergy’ balancing feedback loop, (2) ‘Integration fatigue low monitoring’ reinforcing feedback loop, and (3) ‘Commitment and apathy’ reinforcing feedback loop.