Abstract: Dark energy (DE) dominates the energy density and future expansion of the universe, and its nature is a major unsolved problem for theoretical physics. The equation-of-state and energy density of DE can be constrained observationally by comparing an accurate (< 3% rms) value of the Hubble constant with the known distance to the last-scattering surface of the cosmic microwave background. The Megamaser Cosmology Project (MCP) is now measuring geometric distances to water masers in the nuclei of distant Seyfert galaxies in order to obtain an accurate value of H0 free from the systematic errors associated with "standard candles." I will review the effect of DE on the Hubble constant and present the latest MCP results.