Psychotherapy is sought for diverse problems, and trust in its efficacy has led to increasing parity in insurance coverage for psychological services. But about half of those who begin therapy drop out prematurely, and only about half of those who complete therapy experience significant improvements. Here issues of efficacy measurement and the potential role of personality differences to better guide and assess treatment are explored. It is hoped this knowledge could lead to increased success rates. In terms of outcome measurement, the use and psychometric properties of the Outcome Questionnaire-45 (OQ-45), a popular, brief measure of psychological functioning and change, are assessed. Factor structure is tested in clinical (Study 1) and student (Study 2) samples. Alternative scoring models test whether dimensions of personality drive responses. Using bi-level models, including a total score factor, fit of the intended structure and three- and four-factor personality models was similar. A seven-factor problems model provided the best fit. About half the variance in OQ scores was accounted for by a Big Six personality inventory. The best items for assessing personality attributes in OQ-45 data are noted. Secondly, the relation between personality attributes and clinical presentation and outcomes is explored. Self-report scores on personality attributes predict virtually every life outcome and are highly associated with clinical presentation. Such attributes surely also play a role in treatment, but this association has not been widely studied. In Study 3 the relation of personality attributes to presentation, usage, and outcome is tested in a Couples and Family Therapy clinic sample (N = 222). Neuroticism was strongly associated with initial OQ-45 score, as were other scales (except Openness) to a lesser degree. Being older, more educated, and married predicted attending more sessions, and being older, female, and higher income predicted termination success. Personality scales did not play a role in either outcome. Honesty/Propriety and Neuroticism were, however, associated with steeper decrease in OQ-45 scores over time. / 2015-10-10