The literature is reviewed concerning the pathophysiologic effects of contact-lens wear, the microbiology of contact-lens wear, the change in microflora with contact-lens wear, the contamination of contact lenses and contact-lens products, patient compliance, and corneal interaction with the contact lens. Hypoxia and hypercapnia are the most significant changes in the cornea as a result of contact-lens wear. Changes take place in the conjunctival flora in patients with contact lenses. Compliance of patients and contamination of contact lenses and contact-lens products are significant risk factors. The corneal interaction with the contact lens can overwhelm the protective mechanisms of the cornea, increasing the ability of microbes to adhere to the cornea and progress to microbial keratitis. Some of the factors associated with microbial keratitis are modifiable and should stimulate the contact-lens industry to develop better contact lenses and contact lens products and also permit ophthalmologists to obtain better informed consent from their patients.