The aim of this literature review was to examine pre-treatment child and adolescent characteristics as predictors and moderators of outcome in psychotherapy treatment trials of anxiety and depressive disorders. A literature search was conducted using several databases and resulted in 45 published studies (32 anxiety studies and 13 depression studies) meeting predefined methodological criteria. Ten client demographic (age, gender, ethnicity, IQ) and clinical factors (duration, type of diagnosis, pre-treatment severity, comorbidity) were examined across studies. The majority of findings showed non-significant associations between demographic factors (gender and age) with treatment outcome for both the anxiety and the depression treatment trials. Some important differences between the results of the anxiety and depression treatment trials were found. The majority of findings for the anxiety studies suggest that there are no demographic or clinical factors that predict or moderate treatment outcome. For the depression studies, however, the findings suggest that baseline symptom severity and comorbid anxiety may impact on treatment response. Overall, existing studies of pre-treatment patient variables as predictors and moderators of anxiety and depression treatment outcome provide little consistent knowledge concerning for what type of patients and under what conditions treatments work. Suggestions for future research are discussed.