This chapter establishes the foundational ontological and epistemological principles guiding health service evaluations. It highlights the critical distinction between health service evaluation and broader public policy assessments, focusing on the interplay between objectivist and constructivist paradigms. The chapter emphasizes a pragmatic approach, recognizing the complementarity of reductionist and holistic methodologies in capturing the complexity of health services. Ontology and epistemology are explored in relation to their implications for evaluating healthcare performance, patient experiences and organizational practices. Emerging perspectives, including critical realism and pragmatism, are examined for their potential to bridge philosophical divides and inform actionable, context-sensitive evaluation frameworks. The chapter advocates for pluralistic, theory-led approaches that accommodate both quantitative and qualitative insights, aligning evaluation practices with the dynamic realities of healthcare delivery.