For many years, the study of close double stars seemed somewhat remote from many astronomical problems. The specialists in the field pursued it with enthusiasm, regarding each light or velocity curve as a particular challenge and each system as unusual and interesting in its own right. However, the relation to most other branches of astrophysics sometimes seemed remote. In favorable cases, masses and radii (and hence mean densities) could be obtained and the mass-luminosity relation could be strengthened; even this advantage was partially negated when it was early realized that in many cases the fainter components were notorious violators of the relation established by visual binaries (i.e., systems well separated compared to those discussed here). Various complexities, connected chiefly with interaction effects between the close components, prevented their use for the extremely precise determinations of limb darkening which at one time seemed possible. Other difficulties have prevented the determination, with the precision once comtemplated, of details of internal structure from the apsidal rotation.