Lyme disease, caused by the bacterium Borrelia burgdorferi, is the most common tick-borne illness in the United States. Transmission occurs primarily through the bite of an infected deer tick (Ixodes scapularis). Identification of an erythema migrans rash following a tick bite is the only clinical manifestation sufficient to make the diagnosis of Lyme disease in the absence of laboratory confirmation. The Centers for Disease Control and Prevention recommends a two-tier serologic testing protocol using an enzyme-linked immunosorbent assay initially, followed by the more specific Western blot to confirm the diagnosis when the assay samples are positive or equivocal. The treatment of Lyme disease is determined mainly by the clinical manifestations of the disease. Doxycycline is often the preferred agent for oral treatment because of its activity against other tick-borne illnesses. Preventive measures include avoiding areas with high tick burdens, wearing protective clothing, using tick repellants (e.g., diethyltoluamide [DEET]), performing frequent body checks and bathing following outdoor activities, and instituting environmental landscape modifications (e.g., grass mowing, deer exclusion fencing) to reduce the tick burden. Although there is controversy regarding treatment of post-Lyme disease syndrome and chronic Lyme disease, there is no biologic or clinical trial evidence indicating that prolonged antibiotic therapy is of benefit.