A longstanding research question in the sport psychology literature has been whether a given amount of mental practice prior to performing a motor skill will enhance one’s subsequent performance. The research literature, however, has not provided any clear-cut answers to this questions and this has prompted the present, more comprehensive review of existing research using the meta-analytic strategy proposed by Glass (1977). From the 60 studies yielding 146 effect sizes the overall average effect size was .48, which suggests, as did Richardson (1967a), that mentally practicing a motor skill influences performance somewhat better than no practice at all. Effect sizes were also compared on a number of variables thought to moderate the effects of mental practice. Results from these comparisons indicated that studies employing cognitive tasks had larger average effect sizes than motor or strength tasks and that published studies had larger average effect sizes than unpublished studies. These findings are discussed in relation to several existing explanations for mental practice and four theoretical propositions are advanced.