Immune effector cell (IEC) therapy has revolutionized the treatment landscape of hematologic malignancies with ongoing efforts to translate this success to patients with solid tumors and autoimmune diseases. Cytokines play a vital role in nearly all aspects of the IEC process, including cell manufacturing, efficacy, expansion of the IEC product post-infusion, as well as many of the most common toxicities, such as cytokine release syndrome (CRS) and immune cell-associated neurotoxicity syndrome (ICANS). Despite their known importance, the function of individual cytokines and their potential impact on clinical outcomes are not yet fully understood in many contexts. Future therapeutic advances in the field aim to capitalize on our current understanding of cytokines, employing them to generate longer-lasting and more potent immune effector cells while also mitigating toxicity. This review highlights the role of cytokines in the IEC manufacturing process, their implications for efficacy—including efforts to augment cytokine signaling with next-generation chimeric antigen receptor (CAR) T cells or through the co-administration of exogenous cytokines—and, finally, discusses the crucial role of cytokines in working models of IEC toxicities.