Binary models of symbiotic stars are discussed, analyzing the results of recent IR observations. It is indicated that a binary must have component masses of about 1 solar mass and a period greater than 100 days to exhibit a symbiotic spectrum, and a number of combinations of hot and cool components which would satisfy these criteria are considered. The IR data suggest the existence of two main types, corresponding to Roche-lobe filling and underfilling red giants and characterized, in some cases, by long-period variability and small-amplitude periodic variations, respectively. An ongoing symbiotic-star monitoring program at Asiago Observatory, Tenerife Observatory, and TIRGO is briefly described.