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The concept of applying a lens to the cornea as a refractive appliance was first proposed in the early 19th century. By 1888, glass scleral lenses for the correction of optical defects and irregularities were manufactured and used. New materials, especially soft hydrogel lenses and rigid gas-permeable lenses, became available in the 20th century and allowed comfortable contact lenses to be made in any design needed. By the 21st century, the increasing use of silicone hydrogel lenses to address the oxygen need of the cornea has led to increased worldwide use. Of the 125 million global contact lens wearers, most are female and relatively young. Soft lenses are by far the dominant modality used, with silicone hydrogel lenses taking an increasing share of new fittings, particularly for overnight wear. Microbial keratitis, although relatively uncommon, remains the most serious potential complication for these lens wearers. Ongoing basic research, more powerful antimicrobial agents, and the development of safer lens materials are helping to alleviate this problem.