I-O psychologists must assess the performance of employees in order to evaluate the quality of selection and training programs. This chapter will discuss some of the available options for measuring employee performance. This chapter addresses what is meant by the term “criteria” and what criteria are used for in I-O psychology; what is meant by “actual” versus “conceptual” criteria; the characteristics that good criteria should have; the difference between task performance and contextual performance; the difference between multiple and composite criteria and when each should be used; some of the most important performance outcomes examined by I-O psychologists and the challenges with measuring them; the difference between subjective and objective criteria and the advantages and disadvantages of each; key legal and global issues surrounding the use of criteria in organizations; and the current issues and research in the use of criterion measures.