Researchers and practitioners both acknowledge the integration phase as crucial to the success of merger and acquisition (M&A) projects. However, the integration phase is described as the most complex, uncertain, and dynamic. As these characteristics make both the study and the actual management of integration quite challenging, our knowledge of this activity remains limited. Nonetheless, addressing these questions is highly relevant. To do so, this doctoral thesis proposes an extension of perspectives as it links M&A integration processes with the simple rules program to ask: “How do simple rules interact with the integration process?” Thereby, it follows an embedded, qualitative case‐study design to research the integration phase of an international cross‐border acquisition. Based on 39 interviews, archival data, and observations over a timespan of 18 months, this abductive process study finds that processes during the integration phase are managed via organizational simple rules heuristics. “Located at the heart of strategy”, these semi‐structured rules are developed, used, shared, and reshaped within an organization. In interlinking the simple rules program and the M&A integration process, this dissertation enhances and deepens our overall understanding of the managerial activities in the integration process. It contributes to the PMI audience, with an in depth case study of a large‐scale integration process and a derived model that supports a conception of the PMI phase as a dynamic and complex process. Further, the study offers insights on the management of this process. In doing so, it also addresses the community studying organizational heuristics in 1) determining that simple rules play an important role during strategic change processes; 2) introducing a typology of heuristics that expands the extant ones; and 3) developing a model that informs us about how various actors in an organization work with the heuristics. At the same time, this study offers several practical suggestions to those practitioners who manage or support strategic change processes or are involved in them.