(from the chapter) The goal of personnel selection is to select employees who will succeed in a job, not just over the short-term, but over months and years. Yet, most research and theory in personnel selection does not explicitly consider the effect of time on personnel selection and training. The research that has been done has raised concerns about the utility of predictors used in selection related to declining validity coefficients over time. In this chapter, we discuss selection and training within the context of time. We review prior research and theory on the nature of validity fluctuations over time on person-related and job-related variables. Theories with explicit or implicit temporal components such as theories of typical and maximal performance, skill acquisition theories, theories of dynamic performance, and the simplex process are discussed. We report on a relatively recent reconsideration of time in the research literature, spurred on by the availability of statistical tools that permit the consideration of between-person predictors of within-person change (i.e., interindividual differences in intraindividual change). We conclude by presenting considerations for future research.