The value of deploying and managing ecosystems to innovate, when the innovator has to cope with a high level of uncertainty and a lack of resources, has been well established. Many recent publications focus on what a business ecosystem is and how it differs from platform companies and describe success factors for ecosystems. In our own interactions with executives, we observed that they fully subscribe to the idea that ecosystems are a viable organisational construct to facilitate innovation in conditions of high uncertainty. But they then go on to ask how to start up such an ecosystem, in particular as an incumbent. There is indeed far less written about how this can best be achieved. We have developed a simple model of how to start up and grow an ecosystem and this based on eight in-depth case studies: ensure that the ecosystem leader is credible; bring foundation partners onboard; provide an initial roadmap; clarify what the value is to partners of joining; reduce the entry barriers and the transaction costs; and select partners that can bring a subecosystem along. We also provide some advice on how to scale the ecosystem quickly: have a clear architecture for the ecosystem; avoid the risk of encroaching on partners’ activities; and create incentives for partners to co-invest.