The concept of rarity is critical for understanding patterns of species abundance in the landscape, the ecological processes that drive those patterns, and for conservation of species that become threatened. Multiple definitions of rarity exist in the literature, as well as multiple approaches to the study of rarity. However, no universally accepted definition exists. Without a clear definition of rarity, approaches used to study rarity are inconsistent and communication of results becomes difficult. My research proposes a comprehensive, quantitative definition of rarity. First, I review existing studies to create a comparative analysis of current definitions and approaches to the study of rarity. From this analysis, I develop a conceptual model for a comprehensive approach to define rarity, which forms the foundation of subsequent chapters. Second, using several large-scale vegetation datasets, I develop a quantitative definition of rarity for vascular plants of the Southeastern United States and characterize the patterns of rarity revealed within the region. Finally, I develop a simulation model to examine the effects of an impact such as habitat loss on plants of different forms of rarity. My results indicate that measures of multiple criteria, such as local abundance, geographic range size, habitat preference, frequency, and occupancy, must be included to have a comprehensive understanding of the distributional patterns of species. For vascular plants of the Southeast, rarity is three-dimensional, consisting of abundance, range, and habitat volume. The forms of rarity defined in this analysis are largely congruent with existing lists of threatened and endangered plant species. However, the current analysis reveals species that exhibit distribution patterns similar to threatened species, but that do not yet have conservation status. Finally, simulation results indicate that species of different forms of rarity do respond differently to impacts, indicating that protection of threatened species may need to be approached differently depending on the form of rarity each species exhibits. Taken together, these results indicate that rarity is multidimensional and must be defined as such, and that the information provided by a comprehensive definition of rarity will allow more accurate and efficient development of conservation strategies.