mediation is a publicly available R package that allows both parametric and nonparametric causal mediation analysis. It implements the methods and suggestions in Imai, Keele, and Yamamoto (2010) and Imai, Keele, and Tingley (2010). The package won the 2015 PolMeth Statistical Software Award. In addition to the estimation of causal mediation effects, the software also allows researchers to conduct sensitivity analysis for certain parametric models. Regardless of one's operating system, the software can be installed by typing the following command at R prompt:> install.packages("mediation") This will install the latest version from CRAN. Please make sure that your R is up to date too. The latest version of mediation is version 4.0, and you will need R 2.14. The package is also at the CRAN repository http://cran.r-project.org/web/packages/mediation .  For extensive discussion and details on use, see Imai, Keele, Tingley and Yamamoto (2010).Teppei Yamamoto. (2010). ``Causal Mediation Analysis Using R,'' in Advances in Social Science Research Using R, ed. H. D. Vinod, New York: Springer (Lecture Notes in Statistics), pp.129-154.