Natural History of Spondyloarthritis Study

* INCLUSION CRITERIA: To be eligible for follow-up visits, patients must meet the Inclusion Criteria, but not the Exclusion Criteria. Subjects will provide informed consent and then be evaluated either in the outpatient or inpatient unit of the NIH Clinical Center. * Subjects with known or suspected SpA. * Family members of individuals with known or suspected SpA. Family members will not be asked to submit to bone marrow aspiration or interruption of therapy. * Controls for clinical, cellular, molecular, and biochemical assays, and genetic evaluation will be enrolled. Individuals who undergo phlebotomy specifically to provide a control specimen will be 6 years of age or older, and not pregnant. * Minor healthy volunteers undergoing imaging (SI and/or WB MRI) must be old enough to complete the procedure without sedation. Generally this requires that they be at least 6 years of age, so no healthy volunteers under age 6 will be enrolled. * Pregnant people with SpA are allowed to enrolled. Pregnant people will not be asked to undergo an MRI and will not be asked to submit to skin biopsy or bone marrow aspiration. EXCLUSION CRITERIA: * In the case of minors, unavailability of a parent or guardian to provide informed consent. * Presence of any medical condition that would, in the opinion of the investigators, confuse the interpretation of the study. * Unavailability, or inability to comply with the schedule for follow-up visits. * Children under the age of 2 years old. The vast majority of minors with SpA or suspected SpA will be 6 years of age or older. However, it is possible that children younger than age 6 will be evaluated. If children under age 6 are referred for evaluation, we will ensure that they do not have any acute or chronic comorbidity that would preclude them from being safely evaluated at the NIH Clinical Center. Such patients would be referred to other appropriate pediatric hospitals. Based on estimates of the incidence and prevalence of SpA in the pediatric and adult populations we set the accrual ceiling up to 2000. We hope to enroll up to 100 patients per year. Fecal Microbiota Study INCLUSION CRITERIA: 1. Subjects must be enrolled on another NIH natural history protocol, including either 14-AR-0200 (Studies on the Natural History, Pathogenesis and Outcome of Idiopathic Systemic Vasculitis, P. Grayson PI); 18-AR-0081 (Investigation of the Natural History, Genetics, and Pathophysiology of Systemic Juvenile Idiopathic Arthritis, Adult-Onset Still s Disease and Related Inflammatory Conditions, M. Ombrello PI), or 94-E-0165 (Studies in the Natural History and Pathogenesis of Childhood-Onset and Adult-Onset Idiopathic Inflammatory Myopathies, Lisa Rider PI), and meet the respective protocol s criteria for a diagnosis of systemic vasculitis, systemic juvenile idiopathic arthritis, or dermatomyositis. 2. Subjects with definite SpA enrolled on 11-AR-0223 are eligible for the Fecal Microbiota sub-study. 3. Healthy volunteers and pregnant people enrolled on the above protocols are also eligible. EXCLUSION CRITERIA: 1. Inability to provide informed consent or, in the case of minors, unavailability of a parent or guardian. 2. Presence of any medical condition that would, in the opinion of the investigators, expose subjects to more than minimal risk, or confuse the interpretation of the study. 3. Subjects (including healthy volunteers) less than 4 years of age are excluded due to the challenging practical considerations of collecting stool samples, and because spondyloarthritis is much less common in this age group.