Reviews concepts of aptitude and aptitude-treatment interaction (ATI) and summarizes lessons learned in ATI research on educational treatments to help ATI research on psychotherapeutic treatments. Research design and data analysis recommendations address aptitude distributions, multivariate aptitude complexes, detective work with scatterplots, disattenuation, treatment and therapist characteristics, therapist-client matching, ecological validity, outcome variables, statistical power, aggregation, and person independence. (Author/NB)