Work effort has been a key concept in management theories and research for more than a century. Maintaining and increasing employee effort also is a persistent concern to managers. The goal of the present conceptual and meta-analytic review was to increase clarity and consensus regarding what effort is and how to measure it. First, we reviewed conceptualizations of effort and provided an integrated definition that views effort as a direct outcome of motivation that captures (a) what employees work on, (b) how hard they work, and (c) how long they persist in that work. Second, we identified four main ways researchers have operationalized effort and meta-analytically studied the effects of each operationalization on effort-job performance relationships. For example, measures that assessed multiple dimensions of effort (rho = .37) tended to relate more strongly to performance than measures that focused on only one dimension (e.g., effort intensity) or on effort more generally (rho = .18 to .29). Third, we developed and meta-analytically tested a nomological network to gain a better understanding of effort's antecedents (e.g., intrinsic motivation, rho = .46; performance orientation, rho = .12) and outcomes (e.g., job performance, rho = .34; exhaustion, rho = .04) as well as constructs that appear to overlap with effort (e.g., work engagement, rho = .48; grit, rho = .51). Finally, on the basis of our conceptual and meta-analytic reviews, we delineated an agenda for future research on this central, yet often misunderstood, construct.