Myxomatous valve disease (MVD) is the most common form of cardiac valve disease in the developed world. A small fraction of MVD is syndromic and arises in association with matrix protein defects such as those in Marfan syndrome, but most MVD is acquired later in life through an undefined pathogenesis. KLF2 and KLF4 are zinc-finger transcription factors that are critical regulators of endothelial cell integrity. The KLF2/4 transcription factors mediate endothelial fluid shear responses, including those required to create cardiac valves during embryonic development. In this thesis, we test the role of hemodynamic shear forces and downstream endothelial KLF2/4 in mature cardiac valves. Loss of KLF2/4 from the cardiac valve endothelium leads to a rapidly progressive degenerative valve phenotype, consisting of leaflet thickening, valve cell proliferation and extracellular matrix accumulation, highly resembling human myxomatous valve disease. Interestingly, a similar phenotype was achieved when mouse hearts were heterotopically transplanted into an environment where flow across the mitral valve is removed. Transcriptomic profiling of myxomatous valves revealed increased recruitment of monocytes to the degenerating valve, along with rapid activation of TGFB/SMAD signaling. Whereas removal of circulating monocytes had no effect on myxomatous valve degeneration, concurrent loss of Tgfbr1 led to an improvement in the myxomatous phenotype. Using human mitral valve specimens, we find evidence of reduced endothelial KLF2/4 expression in myxomatous mitral valves. These studies identify hemodynamic activation of endothelial KLF2/4 as environmental homeostatic regulators of cardiac valves and suggest that non-syndromic MVD may arise in association with disturbed blood flow across the aging valve. To begin to parse apart the specific signaling pathway of valve endothelial KLF2/4, we performed CUT&RUN and identified thousands of putative KLF4 binding sites in the mouse heart valve. Altogether this work has identified a critical role for blood flow and endothelial KLF2/4 signaling in maintenance of valve homeostasis in mice.