Purpose: This overview of reviews analyses the existing evidence base of functional family therapy (FFT), which is a manualized, family-based intervention for youth with behavioral problems and their families. FFT has been implemented among youth aged 10-18 at risk of, or presenting with, behavioral problems such as delinquency, violence, substance abuse, sexual perpetration, and truancy. Method: A multipronged search was conducted across 15 databases, 10 websites, and expert contacts in February 2018. Results: The search yielded 159 hits of which 31 were included and critically appraised. Included reviews were published between 1986 and 2018 and the number of included studies ranged from 1 to 18 (including 20-5,344 participants). Main effects of the intervention on core outcomes (recidivism and substance abuse) were modest and out-of-home placement was not reported. Secondary outcomes were also modest but generally positive. Conclusions: Findings demonstrate that overall quality of reviews is low, which makes any certainties about FFT inconclusive; this overview provides a concise, valid, and methodologically sound synthesis of the research into FFT, which requires more rigorous investigation. [For "Call for Revision of Weisman and Montgomery's Review of Functional Family Therapy," see EJ1204964. For "Response To: A Critical Review and Call for Revision of Weisman and Montgomery's Review of Functional Family Therapy by Michael S. Robbins, Charles Turner," see EJ1205029.]