Context • Psychoneuroimmunology is the interdisciplinary study that links behavioral health with the neuroendocrinal system and investigates that link's bidirectional impact on the human immune system. Mechanistic studies have shown how omega-3 polyunsaturated fatty acids (PUFAs), like those found in fish oil, can modulate key pathways involved in inflammation, sympathetic activity, oxidative stress, transcription factors, and inflammatory cytokine production. Objective • The research team intended to investigate the effects that PUFAs have on the brain and the immune system, including the effects of proinflammatory cytokines and oxidative stress, and their therapeutic benefits in major depressive disorder (MDD) and bipolar disorder, either as an alternative monotherapy or a complementary adjunct treatment. Design • A literature search was conducted through PubMed and Google Scholar, with no restrictions on the publication dates or geographically. Setting • The research occurred at research facilities in Washington, DC, and Davis, California. Results • Well-described links between inflammation and MDD and bipolar disorder have been established. Similarly, a highly inflammatory state is a contributing factor to many significant health complications, and omega-3 PUFAs can help treat those issues. Conclusions • The research team concluded that omega-3 fatty acids have therapeutic benefits in the treatment of both MDD and bipolar disorder and are effective as a monotherapy and, particularly, as an adjunct therapy. The efficacy of omega-3 supplementation is clearly useful in promoting better health overall and supplementation should be encouraged in the primary care setting. A meta-analysis exploring an adjunct treatment of supplemental eicosapentaenoic acid or docosahexaenoic acid is likely to yield the greatest benefits to psychiatric conditions and provide an answer to proper dosing regimens. The team also created a chart of the supplements' salient features, demonstrating the overall health benefits of omega-3 fatty acids.