In this paper, we introduce Relational Systems Evaluation (RSE), the focal topic of this volume. RSE is a framework for program planning and evaluation that is theoretically grounded, empirically tested and focused on building evaluation capacity. Theoretically, RSE is rooted in an evolutionary approach to program development and evaluation informed by systems thinking. The application of RSE involves a collaborative partnership approach pairing evaluators or researchers with program practitioners, the use of collaborative modeling and planning tools, and an emphasis on nurturing an Evaluative Thinking mindset. The goal of RSE is to produce more thoughtful and useful evaluations and, ultimately, better programs.