Principal stratification provides an approach to study the effect of an exposure on an outcome within strata defined by the effect of the exposure on some third, posttreatment, variable (Frangakis & Rubin, 2002). There has been more recent interest in using principal stratification to study the extent to which the effect of an exposure on an outcome is mediated by an intermediate variable (e.g., Gallop et al., 2009). In this issue of the "Journal of Research on Educational Effectiveness," Lindsay Page provides evidence that the Career Academies program had substantial effect on subsequent earnings for those for whom the program would change exposure to the world-of-work but not for those for whom it would not change exposure to the world-of-work. The author thinks that principal stratification analyses such as that of Page can be of interest, and he thinks her substantive conclusions are of interest. But if an investigator is specifically interested in studying mediation, then he does not think that principal stratification provides the right approach to do so. Methods for natural direct and indirect effects, not principal stratification, should be used to study mediational processes. Mediation and surrogacy are not equivalent. The approach, definitions, and methods should match the questions of substantive interest. (Contains 3 footnotes and 1 figure.)