Perhaps the central theoretical construct in human resource management today is employee engagement. Despite its centrality, clear theoretical and operational definitions are few and far between, with most treatments failing to separate causes from effects, psychological variables from organizational variables, and internal from external mechanisms. This paper argues for a more sophisticated approach to the engagement concept, grounding it in the vast psychological literature on human motivation. Herein lies the contribution of our paper; we argue that the apparent diversity of operational definitions employed by academics and practitioners can be understood as tentative attempts to draw ever nearer to key motivational concepts, but never quite get there. We review the leading definitions of employee engagement in the literature and find that they are reducible to a core set of human motives, each backed by full literatures of their own, which populate a comprehensive model of twelve human motivations. We propose that there is substantial value in adopting a comprehensive motivational taxonomy over current atheoretical approaches, which have the effect of “snowballing” ever more constructs adopted from a variety of fields and theoretical traditions. We consider the impact of rooting engagement concepts in existing motivational construct for each of the following: (a) theory, especially the development of engagement systems; (b) methods, including the value of applying a comprehensive, structural approach; and (c) practice, where we emphasize the practical advantages of clear operational definitions.