The delivery of large-scale technical systems is achieved through project organizing. The concept of systems integration, with its distinct focus on the systems that projects deliver, is theoretically important as projects become more complex and face significant uncertainty. We reframe systems integration in interorganizational projects as a flexible and adaptive process of making constituent parts of systems work together. This process involves boundary-spanning structures and activities to address emergent complexity and uncertainty (that are both technological and organizational in nature). We discuss implications and highlight areas for further research on projects.