Innate lymphoid cells (ILCs) are a family of immune effector cells that have important roles in host defense, metabolic homeostasis and tissue repair but can also contribute to inflammatory diseases such as asthma and colitis. These cells can be categorized into three groups on the basis of the transcription factors that direct their function and the cytokines they produce, which parallel the effector functions of T lymphocytes. The hierarchy of cell-fate-restriction events that occur as common lymphoid progenitors become committed to each of the ILC lineages further underscores the relationship between these innate immune cells and T lymphocytes. In this Review we discuss the developmental program of ILCs and transcription factors that guide ILC lineage specification and commitment.