Sustainability is one of the most important challenges of our time. Companies are integrating ideas of sustainability in their marketing, corporate communication, annual reports and in their actions. Projects play a pivotal role in the realization of more sustainable business practices, and a growing number of studies link the concept of sustainability to project management. However, as sustainability is understood by instinct, but difficult to express in concrete, operational terms. The evolving concept of sustainable project management is also hard to operationalize. A condition for this operationalization is the availability of an instrument that can be used for the assessment and development of the integration of sustainability in projects and project management. This paper develops the ‘Sustainable Project Management Maturity Model’ as a practical ‘tool’ for the assessment and development of the integration of sustainability in projects. With this maturity model, organizations can translate the abstract and interpretive concepts of sustainable development into practical actions in their projects and monitor their development.