Statistical mediation allows researchers to investigate potential causal e ects of experimental manipulations through intervening variables. Although mediation is common in certain areas of psychology, it is rarely applied in cognitive psychology and neuroscience. One reason for the scarcity of applications is that these areas commonly employ within- subjects designs, and it is only recently that statistical mediation has been worked out satisfactorily for such designs. Here, we draw attention to the importance and ubiquity of mediational hypotheses in within-subjects designs, and we present a free and open source so ware package for conducting Bayesian within-subjects mediation analyses in the R programming environment. e so ware package is easy to use and provides methods for preparing data for multilevel mediation analysis, estimating the mediation model, and summarizing and plotting the results. We use experimental data to illustrate the practice and bene ts of within-subject mediation for theory testing and comparison.