Transcription factor Growth factor independence 1 (Gfi1) is required for multilineage blood cell development, from stem and progenitor cells to differentiated lymphoid and myeloid cells. Gfi1 expression is rapidly induced by cytokines that control both the adaptive and innate immune systems. Gfi1 itself represses the expression of genes implicated in cell survival, proliferation and differentiation. Changes in Gfi1 expression and function have not only been implicated in neutropenia, allergy, autoimmunity and hyperinflammatory responses, but also in lymphoma and more recently in the development of leukemia. In this study, we review how Gfi1 and its paralogue Gfi1b control the development of blood cells, discuss how changes in Gfi1 and Gfi1b function contribute to hematological disease and report on the molecular function of these proteins.