This volume covers the freshwater, aerophytic, and terrestrial green algae of the Ulvophyceae, one of the main classes of green algae. Although most of this diversity is found in the marine environment, a substantial number of species also occurs in brackish, freshwater, and aero-terrestrial habitats. This volume serves as a reference work for identifying these green algae by providing keys, detailed descriptions, and illustrations of the more than 100 European species, along with descriptions of more than 100 non-European taxa. The present study incorporates the latest findings in phylogeny, ultrastructure and morphology for the classification, and delimitation of species. In addition, it revises the taxonomy of ulvophytes, based on new molecular phylogenetic data. One order and one family are resurrected (Chlorocystidales and Chlorocystidaceae), and six families and one genus are newly described (Pseudopleurococcaceae, Binucleariaceae, Planophilaceae, Hazeniaceae, Trichosarcinaceae, and Neocloniaceae). This volume covers the freshwater, aerophytic, and terrestrial green algae of the Ulvophyceae, one of the main classes of green algae. Although most of this diversity is found in the marine environment, a substantial number of species also occurs in brackish, freshwater, and aero-terrestrial habitats. This volume serves as a reference work for identifying these green algae by providing keys, detailed descriptions, and illustrations of the more than 100 European species, along with descriptions of more than 100 non-European taxa. The present study incorporates the latest findings in phylogeny, ultrastructure and morphology for the classification, and delimitation of species. In addition, it significantly revises the taxonomy of ulvophytes, based on new molecular phylogenetic data. One order and one family are resurrected (Chlorocystidales, Chlorocystidaceae), and one order and five families are newly described (Ignatiales, Ignatiaceae, Binucleariaceae, Planophilaceae, Hazeniaceae, Sarcinofilaceae, and Tupiellaceae).