OBJECTIVES: Traditional health economic evaluations of spillover effects are mostly empiric-based assessments of family or informal caregiving based on preference-based measurements. This concept paper explored the term “spillover” used in various academic disciplines and advocates for the use of qualitative methodologies to assess and expand the concept of spillovers in health economic assessments. METHODS: We used Walker and Avant’s (2005) conceptual analysis approach to understand the various definitions, attributes, antecedents, and consequences of spillovers used across disciplines. Google Scholar, PubMed, and OvidMedline databases were searched. We assess the use of qualitative methodologies to date and explore how spillover effects can be best examined through a mixed-method approach. RESULTS: A review of 15 identified articles and white papers revealed that the concept “spillover” is used in sociology, psychology, business/innovation, and economics. Most spillovers in health economics research narrowly assess informal/ family caregiving, while other disciplines consider innovation and learning spillovers, productivity/human capital spillovers, and health care market spillovers. The fields of sociology and psychology employ mixed-methods, qualitative approaches to understand spillovers, while economic-based analyses focus on regression-based analyses, discrete event simulation, and other quantitative-based approaches. CONCLUSIONS: The concept of spillovers in health economics research must be expanded beyond family carers to include health care organizational spillovers inclusive of various stakeholders impacted by health care-related interventions. Qualitative methodologies must be used to inform mixed-method health economic evaluations of spillovers beyond traditional family/informal carer domains to include frontline healthcare clinicians, and other relevant entities impacted by healthcare interventions.