Angiogenesis is the formation of new blood vessels from existing vessels. The formation of new vessels appears to be an early and fundamental process for the evolution of the inflammatory response in synovial joints affected by arthritis. The propagation of new vessels in the synovial membrane allows the invasion of this tissue over the intraarticular cartilage in an adherent fashion. This process appears to support the active infiltration of synovial membrane into cartilage and results in erosion and destruction of the cartilage. This process results in joint damage and ultimately in deformity, as the normal joint architecture and balance of tendons becomes disrupted. Angiogenesis may be assessed in vivo by direct visualization through the introduction of a needle arthroscope using local anesthesia, differential patterns of vascular morphology have been described in seropositive rheumatoid arthritis and seronegative arthritides such as psoriatic and reactive arthritis. At a microscopic level, angiogenesis may be examined in the tissue sections using immunohistochemistry or immunofluorescence. Endothelial cells may also be studied in vitro in culture to examine production of angiogenic growth factors, cell activation, migration, and tubule formation. Finally, synovial biopsy explants may be cultured ex vivo to provide a model simulating the intra-articular milieu.