Cancer is a pathology that strongly associates with metabolic dysregulation and chronic inflammatory responses in the organism. High-fat diets are known to correlate with development of obesity and chronic degenerative pathologies, including type-2 diabetes, cardiovascular disease, and cancer. The specific lipid composition in diet is directly related to the beneficial or detrimental effects of dietary fat on human health, with saturated fat usually considered as less healthy than monounsaturated and polyunsaturated fatty acids. Polyunsaturated fatty acids are known to possess significant anticancer effects, interfering with the proliferation, viability, and invasiveness of multiple types of cancer cells in vitro and in vivo, as well as providing immune modulatory effects that are relevant for decreasing the risk for cancer development and progression. However, different types of fatty acids have different effects depending on their bioavailability, metabolism, and concentrations relative to one another in the organism. The present chapter focuses on available evidence on the effects of polyunsaturated fatty acids on cancer cells in vitro, in in vivo cancer models, and in clinical trials, including their interactions with the immune system and intestinal microbiota. The aim is to provide updated information on the therapeutic potential of these molecules as adjuvants for the prevention and treatment of cancer.