Ability, Motivation and Opportunity (AMO) theory has been adopted extensively to potentially explain the complex relationship between how people are managed and subsequent performance outcomes. Specifically, the theory suggests some combination of an individual’s ability (A), motivation (M) and opportunities (O) can give us a measure of an individual’s performance (P) (expressed as AMO = P). AMO theory is concerned with individual characteristics as independent variables, however, in its application in the HRM field, researchers have supplemented these independent variables for HR practices and policies, resulting in at least two different incarnations of the AMO model. Further compounding this issue, AMO theory has seldom seen empirical testing, and there is significant lack of consistency in definition and selection of variables. In this chapter, the authors develop an argument that AMO theory is poorly defined and tested, and its appeal is that it can be adapted to suit almost any HRM study.