I reviewed studies of small mammals in order to assess the methodological constraints in documenting nest mortality in the wild, to identify the major causes of nest mortality, and to consider the impact of nest mortality on population dynamics. Population-level studies are not sufficiently intensive to monitor nest mortality accurately, and the best data are from individual-level studies. The most reliable estimates of nest mortality average over 50% in most populations and range from less than 30% to over 90%. The potential for nest mortality to have a significant impact on population dynamics is high. However, the extent of nest mortality has rarely been studied, and the actual impact of nest mortality on population dynamics is not clear. Multiple causes, including predation on adults and nestlings, infanticide, food constraints, and weather-related events appear to affect nest mortality at any given time, with no single cause predominating. It is suggested that future long-term studies with a focus on nest mortality be conducted, with mortality partitioned to distinguish between losses due to the mortality of adults and the direct mortality of nestlings. Such studies are needed to adequately assess the role of nest mortality in population dynamics.