An overview of the evolution of different fossil fuel sources, including coal, conventional and unconventional (tight) oil, oil sand, conventional natural gas, unconventional shale gas, and coalbed methane. The chapter provides the data on energy flows and evaluates the global energy consumption for understanding the magnitude of these developments. The chapter also presents the traditional water–energy concept; the amount of water withdrawal and consumption for fossil fuel exploration in the recovery stage (coal mining, oil and gas drilling, mining, hydraulic fracturing, oil enhancement), processing (coal washing, oil refinery), conversion (electricity production), and post-conversion (waste disposal). The chapter examines the water intensity metrics by normalizing the water volume-per-energy unit of electricity, with an emphasis on the distinction between water withdrawals and water consumption, and considering the complete lifetime cycles of water extraction for energy exploration, processing, and generation.