This chapter explores the evolving landscape of evaluating complex interventions in healthcare, driven by the increasing need for evidence-based resource allocation and improved patient outcomes. It discusses the defining characteristics of complex interventions, such as multiple interacting components, contextual adaptability, and non-linear causal pathways, and contrasts them with simpler technologies. Drawing from frameworks like the MRC guidance and RE-AIM, it outlines the rationale, challenges, and methodological approaches for their evaluation, including formative and summative assessment, process evaluation, and both experimental and quasi-experimental study designs. Special attention is given to integrated care as a paradigmatic example of complexity in healthcare. The chapter emphasizes the importance of context, fidelity, and stakeholder perspectives in designing and reporting evaluations that are robust, transferable, and policy-relevant.