Anxious and depressive disorders share common etiology and respond to similar treatments (Garber & Weersing, 2010). Transdiagnostic protocols may aid in treatment efficiency and reduce the burden of dissemination. Little is known regarding whether therapists are able to implement these treatments equally well across the range of clinical profiles they are intended to target and whether participants are equally engaged. Youth (ages 8-16) with diagnoses of an anxious or depressive disorder were randomized to either brief behavioral treatment (BBT; N = 95) or usual care. Therapists rated adherence and participant engagement. Audio recordings of treatment sessions were also coded for adherence. Aim 1: Youth psychopathology severity, parental psychopathology, and family conflict were examined as predictors of adherence. Higher levels of psychopathology and conflict were hypothesized to predict lower levels of adherence. Aim 2: Psychopathology and conflict were examined as predictors of engagement. Higher levels of psychopathology and conflict were hypothesized to predict lower levels of engagement. Aim 3: Engagement was examined as a predictor of adherence. Higher levels of engagement were hypothesized to predict better adherence both within and across subsequent sessions. Aim 1: As hypothesized, anxiety severity and comorbid depression predicted lower levels of therapist-reported adherence. In contrast with hypotheses, parental psychopathology and family conflict did not predict adherence. As hypothesized, in analyses of observer-reported adherence, higher levels of youth psychopathology, parental psychopathology, and family conflict predicted less coverage of core components of treatment and more time structuring session. Aim 2: As hypothesized, higher levels of youth psychopathology, parental psychopathology, and family conflict predicted lower levels of engagement. In-session engagement was predicted by youth psychopathology whereas homework completion and parental involvement were predicted by parental psychopathology and family conflict. Aim 3: As hypothesized, higher levels of engagement were associated with higher levels of adherence within session, but not between phase of treatment. Results supported youth symptom severity and comorbidity as predictors of in-session processes (e.g., adherence) and parental psychopathology and family conflict as predictors of between-session processes (e.g., homework completion). Results from the current study are relevant in identifying families who may be at risk of receiving suboptimal treatment.