-Mark-recapture models are classified according to requisite assumptions about population closure. The resulting classes of models are briefly discussed and the experimental situations to which they apply are described. Model assumptions are presented, with emphasis being placed on those which are most likely to be violated in avian population studies. Comments are provided on experimental design, and previous ornithological studies in which the various models have been used are briefly reviewed. Special attention is devoted to the Jolly-Seber model which was developed for open populations and which perhaps has the greatest potential applicability to detailed, long-term population studies. A number of examples of the use of the JollySeber model with avian mark-recapture data are presented in summary fashion. The literature of field ornithology is much more extensive than that dealing with field studies of other terrestrial vertebrates. However, mark-recapture methods of estimating population size have seen only limited use in the ornithological literature, but are frequently employed in published studies of mammals, reptiles and amphibians. For example, reviews of small mammal population estimation are dominated by mark-recapture methodologies (e.g., see Smith et al. 1975), while such methods are barely mentioned in methodological reviews of avian population estimation (Kendeigh 1944, Berthold 1976, Shields 1979). We believe that the neglect of mark-recapture methods in avian studies results from the generally high visibility and audibility of birds and the relative ease with which they can be directly enumerated. This ease of enumeration has naturally and justifiably resulted in an emphasis by ornithologists on estimation methods that involve actual counts of birds (or nests), which are then expanded in various ways to estimate total population size or density. However, no single population estimation method is universally appropriate for avian studies, and there is still much debate about census methodology (see J. T. Emlen 1971, Berthold 1976, Shields 1979, this symposium). We do not suggest that mark-recapture methods will generally provide a preferable alternative to direct observational methods of estimating avian population size. However, we do believe that mark-recapture studies can provide estimates that are useful in assessing the appropriateness of other estimation methods, and that they may provide the most reasonable means of estimating population size in some situations. In this paper we attempt to introduce the subject of mark-recapture methods and to briefly review their use in previous avian studies. We will be 1 U.S. Fish and Wildlife Service, Migratory Bird and Habitat Rewarch Laboratory, Laurel, Maryland 2081 I. concerned only with the estimation of population size, and will thus omit discussion of models for estimating survival rate from band recoveries of dead birds (e.g., see North 1978 and reviews in Taylor 1966, Seber 1973, Brownie et al. 1978). Our discussion will be introductory and brief, and we urge the interested reader to consult the excellent general reviews of mark-recapture models and their associated literature provided by Cormack (1968, 1979) and Seber (1973). Mark-recapture models can be usefully classified according to their requisite assumptions about population closure. We define a closed population as one which remains unchanged over the period of investigation and which is thus not influenced by mortality, recruitment, or migration (both emigration and immigration) during this time. An open population is one that does change over the period of investigation as a result of either mortality and emigration, recruitment and immigration, or both sets of factors.