The North Carolina Natural Heritage Program's Classification of the Natural Communities, now in its fourth edition, is intended as a reference book describing the state's biodiversity at the community and ecosystem levels. It describes natural patterns of organisms that typically co-occur together in a particular kind of environment. The classification draws on years of use and data collection by North Carolina's Natural Heritage Program, the extensive vegetation plot data of the Carolina Vegetation Survey, and the National Vegetation Classification. The classification units are intended for detailed conservation planning at the natural community level, for representing the diversity of ecological function, for characterizing the habitats of species, and for contributing to the conservation of the myriad species that are too poorly known to track individually. The 343 community subtypes are grouped into types and are nested within 30 broader ecological themes that have been recognized in North Carolina for decades, retaining continuity with previous editions. Material to aid in identification of the communities includes concept statements, dichotomous keys, and descriptions of the distinctions from closely related communities. Extensive descriptions of each community and each theme include sections on physical settings, soils, hydrology, vegetation, range, abundance, and ecological dynamics, using scientific nomenclature and terminology. Crosswalks to the National Vegetation Classification and to NatureServe's ecological systems and listings of rare plant and animal species associated with each community provide additional functionality. While technical in style-mostly providing scientific names instead of common names-this book can offer the educated layperson and scientist alike an expanded appreciation of the diversity of natural settings in North Carolina. It is expected to remain a primary reference for years to come.