This cross-sector guide, informed by a rapid evidence review and expert consultation, explains the foundations of Theory-Informed Implementation and Process Evaluation (TIIPE) and sets out best-practice principles in designing and delivering TIIPE. The guide supports evaluators to develop and test program theory, develop and test implementation theory, design TIIPE alongside impact evaluation, and generate learning about how program work for different groups.

TIIPE is an approach that uses program and implementation theory to systematically explore how programmes are delivered, and how and why outcomes are achieved (or not). By exploring the mechanisms that drive change, and the contexts in which this change occurs, TIIPE also enhances the understanding of causal pathways, and provides actionable insights for improving, replicating and scaling programs. This guide can be used to assess and inform program design and implementation to determine readiness for impact evaluation, and enhance the learning generated through an impact evaluation, typically a randomised controlled trial or quasi-experimental design.

The guide is structured around:

• program theory: developing and refining a Theory of Change
• implementation theory: understanding how delivery works in practice
• integrating program and implementation theory
• application across evaluation stages.