This chapter is about measuring task performance (i.e., the technical proficiency part of job performance) in personnel selection research. When evaluating the validity of selection tests and procedures, the accuracy of these validity estimates depends in turn on the accuracy of criterion performance measurement. Accordingly, there is considerable motivation in selection research to obtain reliable and accurate criterion scores for job incumbents participating in the research. Our chapter covers the task performance criterion “space.” Chapter 21 in this volume describes citizenship performance criteria. Specific topics covered in this chapter are (a) relatively objective criterion measures, such as work samples, job knowledge tests, and production rates; (b) subjective measures (i.e., ratings of performance), including different rating formats and rater training strategies; (c) dimensionality of job performance; and (d) validity estimates against task performance for several predictor constructs (e.g., ability, personality, etc.).