Biological treatment is effective but infrequently used for oil and gas produced water. To date, physical–chemical treatment methods have been favored due to the smaller space requirements and operational simplicity. Changing regulatory requirements and increased interest in recycling and beneficial reuse have led to increased interest in biological treatment. To elucidate its potential role, we reviewed and summarized 59 studies on the biological treatment of produced water. Oilfield produced water was predominantly studied (> 50%). More studies using real produced water were from China than from any other country (37%). Real produced water was used in most studies (73%). Studies were predominantly bench-scale experiments (69%). Fixed-film reactors were most prevalent (27%). Water quality of produced waters treated was variable; median total dissolved solids (TDS) was 28,000 mg L−1 and median chemical oxygen demand (COD) was 1125 mg L−1. Inhibition by salinity was variable according to the treatment system and study design, but efficacy generally decreased when TDS was above 50,000 mg L−1. For studies treating real samples, average COD removal was 73% when TDS was less than 50,000 mg L−1, and 54% when TDS was greater than 50,000 mg L−1. Key issues were microbial acclimation, toxicity, biological fouling, and mineral scaling. Finding an inoculum was not problematic as microorganisms capable of degrading hydrocarbons were isolated from various environments. Treatment performance was better where synthetic produced water was used in lieu of real samples. Biological treatment is promising for producing effluents suitable for reuse, particularly where it functions as part of a larger treatment train.