Workers often identify understaffing as a major stressor in their work lives. Despite this, relatively little conceptual and empirical work on understaffing exists. This paper describes a new, multidimensional conceptualization of understaffing, specifying that there are three dimensions underlying the understaffing domain: severity of (under)staffing, type of resource shortage, and length of exposure. Drawing upon theory and research on workplace demands and self-regulation, we further argue that different types of understaffing are differentially related to workplace outcomes. After specifying what understaffing is, we then compare and contrast understaffing with conceptually similar or related constructs in the industrial-organizational/organizational behavior (IO/OB) literature to assist in explaining what understaffing is not. Finally, we address practical issues in the study and measurement of understaffing. Implications for future research and theory are discussed.