Microscopic, unicellular, free-living green algae are found in desert microbiotic crusts worldwide. Although morphologically simple, green algae in desert crusts have recently been found to be extraordinarily diverse, with membership spanning five green algal classes and encompassing many taxa new to science. This overview explores this remarkable diversity and its potential to lead to new perspectives on the diversity and evolution of green plants. Molecular systematic and physiological data gathered from desert taxa demonstrate that these algae are long-term members of desert communities, not transient visitors from aquatic habitats. Variations in desiccation tolerance and photophysiology among these algae include diverse evolutionary innovations that developed under selective pressures in the desert. Combined with the single embryophyte lineage to which more familiar terrestrial green plants belong, multiple desert green algal lineages provide independent evolutionary units that may enhance understanding of the evolution and ecology of eukaryotic photosynthetic life on land.