Although information systems development (ISD) projects are critical to organizations and improving them has been the focus of considerable research, successful projects remain elusive. Focusing on the cognitive aspects of ISD projects, we investigate how and why mutual understanding (MU) among key stakeholder groups (business and information technology managers, users, and developers) changes within and across projects, and how it affects project success. We examine relationships among project planning and control mechanisms; sensegiving and sensemaking activities by, and MU among, these stakeholder groups; and project success. Combining deductive and inductive approaches for theory building, we develop an initial model based on the literature and then modify it based on the results of a longitudinal embedded mixed-methods study of 13 projects at 2 organizations over a 10-year period. The results provide insights into the development of MU within projects, including (1) how MU changes during projects as a result of cognitive activities (sensegiving and sensemaking); (2) how planning and control mechanisms (and the associated artifacts) affect these cognitive activities; (3) how MU, and achieving it early in the project, affects success; and (4) how stakeholder engagement (in terms of depth, scope, and timing) affects the relationships in (1) and (2). The results also indicate that project management mechanisms, stakeholder engagement, and MU may change (either improve or deteriorate) across projects, depending on the disagreements among stakeholders in previous projects, the introduction of new project elements in subsequent projects, and the reflection on previous projects.