This article presents the authors' rejoinder to the comments of Howard Bloom, Derek Neal, and Mike Seltzer on their article. Their commentary focused foremost on the advantages and disadvantages of using Options A, B, or C in using instrumental variables in multisite trials. Of interest are the quantities one can estimate and test in each case, strength of assumptions required, and robustness to outliers, and strategies for probing assumptions. The other key theme regards approaches to enrich the study of mechanisms whereby programs come to have effects. A central concern in the authors' article is the interpretation one can make of the average effect of program participation in each site; the overall average effect of program participation; and the between-site variance in site-specific effects. (Contains 1 table.)