Hydropower is the oldest source of renewable mechanical power and largest source of renewable electricity globally accounting for close to 17% of global electricity generation although its share of contribution has been diminishing gradually. Hydropower generation is a mature, reliable, predictable, and socially and economically competitive with high efficiencies greater than 90% in terms of water to wire conversion. There are different types of hydropower plants based on different criteria like size, application, and use of storage. Based on size they can be o micro, mini, small, or large hydropower plants, while based on storage, they can be either dam and reservoir plants or run-of-river stations. Different hydraulic turbines including the Pelton, Francis, and Propeller turbine are used in hydropower generation. Hydroelectric power plants have environmental impacts that should be considered and mitigated before development. The benefits of hydropower include low unit cost, low emissions, ease of control, and storage. However, hydro has challenges like large-land requirements for storage, soil erosion, ecosystem disturbance, and seasonal variability. It is technically feasible to combine hydropower with wind, solar, and other sources to develop a steady and affordable grid electricity mix. Hydropower plants with storage have additional benefits like flood control, water supply for industrial and domestic use, and power production for a sustainable grid in terms of flexibility and cost of power. Hydropower generally has higher initial investment cost but their long life span and lower operation and maintenance cost make hydropower competitive and sustainable option in green power generation.