RecruitingNatural History and Development of SpondyloarthritisBackground: \- Spondyloarthritis (SpA) is a group of bone and joint disorders that may cause back and joint pain and stiffness. In some cases, SpA can lead to abnormal bone growth affecting the joints and spine. Some patients have SpA without ever developing these growths, while others develop them after only a few years. Researchers are interested in studying people with SpA and their relatives to determine which people are more likely to develop more severe conditions. Objectives: \- To identify symptoms and medical tests that can help determine whether a person with SpA is at risk for developing more severe forms of the disease. Eligibility: * Individuals of any age who have been diagnosed with SpA. * Healthy volunteer relatives (at least 6 years of age) of the individuals with SpA. Design: * Participants will be screened with medical records and family medical histories, and will be invited to the clinical center for the study. * Participants with SpA will have a physical exam and medical history, including a study of joint movement, blood and urine tests, and questionnaires about pain and quality of life. * Participants with SpA will have imaging studies, including magnetic resonance imaging (MRI). Other samples such as skin tissue and bone marrow may also be collected for study. * Healthy volunteers will provide a blood sample and cheek cell samples. * No treatment will be provided, although treatment options will be discussed....