The diversified class of immune cells known as innate lymphoid cells (ILCs), which lack altered antigen-specific receptors, is essential for the maintenance of immunological homeostasis, tissue repair, and protection against pathogens. They are the innate counterparts of CD4+ Th cells. The plasticity of ILCs presents a potential for therapeutic interventions, as understanding the signals that drive their functional changes could lead to novel treatments for various diseases. This chapter examines the division of ILCs into ILC1, ILC2, and ILC3 subsets according to the production of cytokines and the expression of transcription factors, as well as the fundamental characteristics, heterogeneity, and plasticity of ILCs. The dynamic nature of ILCs allows them to adapt to various environmental cues, contributing to their involvement in a wide range of physiological and pathological processes. We delve into the important roles of ILCs in immune disorders, where their dysregulation is linked to conditions such as tissue repair, inflammation, asthma, allergy, cancer, and autoimmune diseases. The chapter also examines the dual roles of ILCs in cancer, where they can either promote or inhibit tumor growth, depending on the context. Here, we present the current state of research regarding ILCs, their functions in both health and disease, and their possibilities for therapeutic uses.