Need is a pivotal concept within health systems internationally given its driving force in health care policy, development, and delivery at population and individual levels. Needs assessments are critical activities undertaken to ensure that health services continue to be needed and to identify new target populations that demonstrate unmet need. The concept of need is underpinned by varied theoretical definitions originating from various disciplines. However, when needs are assessed, or health interventions developed based on need, little, if any, detail of the theoretical or conceptual basis of what is being measured is ever articulated. This is potentially problematic and may lead to measurement being invalid and planned health services being ineffective in meeting needs. Seldom are theoretical definitions of need ever compared and contrasted. This critical review is intended to fill this gap in the literature. Interpretations of the concept of need drawing from areas such as psychology, social policy, and health are introduced. The concept and relevance of unmet need for health services are discussed. It is intended that these definitions can be used to operationalize the term "need" in practice, theoretically drive needs assessment, and help guide health care decisions that are based upon need.