In the medical and mental health literature, the concept of functioning is commonly used and refers to different terms and definitions. The purpose of this study was to clarify the term “functioning” and examine instruments that evaluate human functioning. The Walker and Avant method was used to conduct a concept analysis. This analysis identified four main attributes that allow human functioning to be operationalized and support development of a measure of functioning in mental health. The manner in which occupational therapists and occupational scientists conceptualize human functioning, enriches the understanding of this concept in mental health.