Many factors, both biotic and abiotic, impede the success of reintroductions. Among these is the difficulty that small populations have in surviving and thriving. For many species of animals and plants, populations at low density or of small size suffer from a weak or even a negative growth rate, either of which increases their extinction risk; this is called the Allee effect. As reintroductions are almost invariably characterized by small propagule sizes, the importance of the Allee effect must be considered in order to assess its potential impact on the probability of success in reintroductions. In this paper, we highlight the ubiquity of the Allee effect, the importance of this process as predicted by theoretical models, and its potential role in the failure of previous reintroduction attempts. We emphasize that although previous reintroduction programs have shown that release size is paramount to the success of reintroductions, no link has yet been formally demonstrated between Allee effects and reintroduction success. We encourage researchers in ecology to further investigate the role of the Allee effect on the probability of reintroduction success, using available data on comparable processes such as invasive species and the propagation of biological control agents where possible. We also urge biodiversity managers to consider this potential impediment to the survival, increase, and spread of small groups of reintroduced individuals in conservation programs.