We propose a systems integration model for the delivery of complex infrastructure projects. We argue that the client is ultimately accountable for systems integration in major projects, setting out the responsibilities to ensure that systems integration is successfully accomplished to achieve desired outcomes. From the Crossrail case, we draw seven lessons for clients, to: 1) manage programme delivery as an integration activity; 2) actively manage systems integration; 3) ensure authority to make decisions; 4) maintain configuration control; 5) plan for a lengthy testing and commissioning phase; 6) appreciate supply chain products may be part of unaligned global R&D and development programmes; and 7) do final integration only when there is something to integrate. Central to our argument is the idea that on such complex projects, the client cannot outsource systems integration and thus needs to recognize they retain accountability, though roles and responsibilities can be assigned to the delivery partner, supply chain, chief engineer and/or contracted systems integration firm. A key question for the client at the outset is how to distribute interface management and systems integration responsibilities while retaining accountability and oversight. Rather than managing through contracts, budgets and schedules, we suggest a changed approach with priority given to managing integration, and contracts, budgets and schedules that support and incentivise this.