This primer provides a synthesis of guidance specifically aimed at carrying out Theory of Change in processes in a GEF context. The document is part of a growing suite of STAP documents intended to support the design of interventions within GEF's goal to apply leading practices to deliver transformational change. The primer provides a brief overview of the origin of Theory of Change; defines what is a Theory of Change; explains why developing and carrying out a Theory of Changes is necessary; describes when to do a Theory of Change; and, provides a succinct guide on how to do a Theory of Change. The primer also makes a distinction between Theory of Changes for projects and programs - given their distinct cycles. In addition, the document provides, examples of Theory of Change, and sources of further information. The primer is accompanied by a short literature review and annotated bibliography. The primer and its supplement are listed below.