This chapter discusses defining performance and choosing a measurement approach. It defines performance as behaviors rather than results or traits. The main determinants of performance are declarative knowledge, procedural knowledge, and motivation. Performance should be measured on both task and contextual dimensions. The three main approaches to measuring performance are the trait, behavior, and results approaches. The most appropriate approach depends on factors like whether behaviors or results can be directly observed and the link between behaviors and outcomes. Choosing the right measurement approach is important for accurately evaluating performance and addressing any issues.