Acanthamoeba is a small, free-living amoeba which is characterized by a feeding and replicating trophozoite and dormant cyst stage. The resistance of acanthamoeba cysts to extremes of temperature, disinfection and desiccation accounts for the ubiquitous distribution of the organism in our environment. Acanthamoeba produces two distinct forms of disease in man. Granulomatous amoebic encephalitis (GAE) is a chronic and invariably fatal infection occurring in immunocompromized persons. Since GAE was first recognized in 1971, approximately 50 cases have been reported worldwide. In contrast, keratitis due to Acanthamoeba affects previously healthy persons and occurs with far greater frequency. Since the disease was first described in 1973, many hundreds of cases of acanthamoeba keratitis have been reported worldwide. Approximately 80–90% of these have been in wearers of contact lenses. Acanthamoeba keratitis is perhaps the most difficult ocular infection to manage successfully and can result in permanently impaired vision or blindness. Acanthamoeba keratitis continues to be reported with increasing frequency, at least 90 cases having occurred in the United Kingdom since 1974 and, of these, 63 were recorded in the past 3 years.