Tail-anchored or C-terminally-anchored proteins play many essential roles in eukaryotic cells. However, targeting and insertion of this class of membrane protein has remained elusive. In this study, we reconstitute insertion of tail-anchored proteins into microsomes derived from Saccharomyces cerevisiae. Using this approach, we are able to genetically manipulate the composition of the microsomes in order to address the question of which components of the endoplasmic reticulum (ER) are required for this process. We show that tail-anchored protein insertion is not dependent on the classical SEC translocation machinery but rather occurs via an ATP-dependent pathway involving at least one novel membrane protein factor. We further demonstrate that the specificity of this pathway is conserved between yeast and mammals.