Significant therapist variability has been demonstrated in both psychotherapy outcomes and process (e.g., the working alliance). In an attempt to provide prevalence estimates of "effective" and "harmful" therapists, the outcomes of 6960 patients seen by 696 therapists in the context of naturalistic treatment were analyzed across multiple symptom and functioning domains. Therapists were defined based on whether their average client reliably improved, worsened, or neither improved nor worsened. Results varied by domain with the widespread pervasiveness of unclassifiable/ineffective and harmful therapists ranging from 33 to 65%. Harmful therapists demonstrated large, negative treatment effect sizes (d= -0.91 to -1.49) while effective therapists demonstrated large, positive treatment effect sizes (d=1.00 to 1.52). Therapist domain-specific effectiveness correlated poorly across domains, suggesting that therapist competencies may be domain or disorder specific, rather than reflecting a core attribute or underlying therapeutic skill construct. Public policy and clinical implications of these findings are discussed, including the importance of integrating benchmarked outcome measurement into both routine care and training.