Reaction time is believed to be a good indicator of the speed and efficiency of mental processes and is a ubiquitous variable in the behavioral sciences. Despite this popularity, there are numerous issues associated with using reaction time (RT), specifically in differential and developmental research. Here, we identify and focus on two main problems-unreliability and sensitivity to speed-accuracy interactions. The use of difference scores is a primary factor that leads to many RT measures having demonstrably low reliability, and RT measures in general often do not properly account for speed-accuracy interactions. Both factors jeopardize the validity and interpretability of results based on RT. Here, we evaluate conceptually and empirically how these issues affect individual differences research. Although the empirical evidence we provide are primarily within the domains of attention control and task switching, we highlight examples from various other areas of psychological inquiry. We also discuss many of the statistical and methodological alternatives available to researchers conducting correlational studies. Ultimately, we encourage researchers comparing individuals of differing cognitive and developmental levels to strongly consider using these alternatives in lieu of RT, specifically RT difference scores. Public Significance Statement This review identifies and discusses the problems with the use of RT, particularly RT differences, in assessing how individuals differ from one another. Given these problems, a variety of conclusions and theoretical accounts stemming from RTs in individual differences studies may be misinformed. Examples include the efficacy of some clinical techniques, the measurement of racial bias, and the measurement of attention.