Proteorhodopsins (PRs) are membrane-embedded, retinal-containing proteins that function as light-driven proton pumps. Since their discovery in 2000 in uncultured marine bacteria, PRs have been detected in numerous bacteria, archaea, and microbial eukarya. PRs have now been detected in diverse habitats, including marine, brackish, and freshwater environments; leaf surfaces; and soil crusts. Their widespread distribution and relatively simple single-molecule design suggests them as the earliest light-energy transducing proteins to have evolved. Currently, PRs represent the simplest biological means to convert light energy into chemical energy.