Introduction to the special section: efficacy and effectiveness in studies of child and adolescent psychotherapy.

This article introduces a special series on "transporting" studies of the efficacy of psychotherapy into studies of their effectiveness. Models of the relationship between efficacy and effectiveness are described. Traditional linear models of the phases of scientific expansion suggest that findings from efficacy studies can be transported into broader population samples only after a prescribed series of steps have been followed, with the ultimate goal of implementing treatments that have broad public health implications. An alternative 3-dimensional model is described. This new model posits as its organizing principle flexible movement between dimensions of efficacy and effectiveness across multiple axes.