<p>Innate lymphoid cells (ILCs) are a diverse group of immune cells that possess many effector functions of T cells but lack somatically generated receptors. They have crucial roles in early immune responses and in maintaining tissue integrity. We review ILC developmental pathways and progenitors in mice, with a particular focus on adult bone marrow but also addressing fetal liver. We present recent insights into the earliest steps of ILC specification, as well as new evidence for developmental pathways that generate two functionally distinct types of natural killer cells. The anatomical locations where ILC progenitors are identified are outlined and evidence supporting ILC development in tissues examined. In addition, key transcriptional regulators that support ILC development are discussed. Although ILC and T-lineage cells use many of the same transcriptional controllers during development, we present new evidence indicating ILC transcriptional programs diverge from T-lineage programs at the earliest known steps of ILC lineage specification.</p>