Lyme borreliosis continues to be a significant public health problem. Because of good public education, the incidence of Lyme arthritis and other late-stage symptoms has not increased in tandem with the increases in early infection. There have been major advances in the understanding of the pathogenesis of Lyme arthritis. The understanding of mechanisms of arthritis in Lyme borreliosis may also shed light on the mechanisms of disease in other infectious and noninfectious arthritides.