This review of existing estimates confirms an upward trend in the number of facilities, beds, and residents in residential care alternatives to nursing homes, often collectively referred to as "assisted living." Estimates vary substantially, however, depending on the methodology and the type of data used. Key sources of differences are population included (i.e. aged, all ages); definition used to identify assisted living, and for population-based surveys that include both community and facility settings, survey-specific definitions of "facilities" and whether settings identified as assisted living are limited to those meeting the survey-specific facility definition. Greater disagreement exists with respect to trends in the number of nursing homes and users, even between estimates from the same data source.