The ETS family of transcription factors is known to play an essential role in hematopoietic and vascular development. One such factor that is widely expressed in all vascular beds and almost all hematopoietic lineages, extending from long term hematopoietic stem cells to terminally differentiated peripheral blood cells, is Fli1. Global deletion of Fli1 leads to embryonic lethality at E11.5 due to dramatic hemorrhaging caused by poor vascular integrity and platelet dysfunction. The role of Fli1 in adults has been explored in several different cells types ranging from megakaryocytes to B and T cells. It has also been implicated in transcriptional regulation of hematopoietic stem and progenitor cells through combinatorial analysis, however its exact contribution to stem cell maintenance and function remains unclear. We hypothesized that Fli1 plays a dual role in regulating both the seed and the soil – specifically hematopoietic stem cell (HSC) function in a cell autonomous manner as well as the niche required for nurturing these cells. In the first study, we focused our attention on the cell autonomous function of Fli1. We found that global deletion of Fli1 in adults leads to lethality as a result of complete peripheral blood failure in addition to aberrant vasculature. Using the cre-lox system and various transplantation strategies, we identify Fli1 as one of the critical regulators of adult hematopoietic stem cell function. Specific deletion of Fli1 in the hematopoietic compartment alone is sufficient to induce significant reduction in peripheral blood counts, accompanied by hemorrhage, resulting in lethality. On further inspection, Fli1Δ/Δ HSCs are unable to expand ex-vivo or engraft in a competitive setting. Additionally, Fli1 is essential for hematopoietic reconstitution post radiation and its deletion abolishes the ability of stem cells to reconstitute the bone marrow and contribute to peripheral blood lineages. ChIP-seq analysis demonstrates Fli1 occupancy in a majority of essential hematopoietic gene enhancers and promoters. By correlating Fli1Δ/Δ HSC RNA-seq analysis with the ChIP occupancy data we can conclude that Fli1 is, in fact, one of the master regulators of the HSC transcriptional program essential to maintain the balance between self-renewal and differentiation. Taken together, our results indicate that Fli1 is required for maintenance of HSC homeostasis and function, making it one of the unique transcription factors to play a critical role in HSC function in adults as well as during development. In the second set of studies, we interrogated the role of Fli1 in the endothelial niche and more specifically in the maintenance of endothelial homeostasis. We found that Fli1, by itself, is not essential for maintaining endothelial fate during homeostasis or under stress. We hypothesized that this may be due to redundancy between Fli1 and Erg, another ETS factor that shares significant homology with Fli1. Endothelial specific deletion of both Fli1 and Erg results in lethality, 12 days post induction due to disruption of vascular homeostasis and the formation of multiple thrombi in the liver. Fli1 and Erg deletion results in a reduction in a variety of vascular genes in multiple vascular beds in addition to organ specific vascular signatures. The disruption of vascular homeostasis is accompanied by a significant reduction in platelet numbers in peripheral blood. In vitro analysis of the endothelial deletion reveals a complete loss of vascular identity with significant reduction in VE-Cadherin and CD31 expression accompanied by a concomitant increase in mesenchymal marker α-SMA. Taken together, these studies uncover a critical role for Fli1 in maintaining homeostasis in both endothelial cells as well as hematopoietic stem cells.