The clinical properties of hydrogel and rigid gas-permeable contact lenses for extended wear are examined, with emphasis on oxygen permeability and transmissibility, so that clinicians may select lenses likely to minimize corneal edema or may predict the amount of edema likely with a given lens. Lens deposit formation and related problems are also described, as are lens care systems, particularly their limitations with respect to preventing infections. The advantages, disadvantages, and limitations of lens cleaners are discussed, and recommendations for use are provided.