Juvenile Idiopathic Arthritis (JIA) is not a single disease. It is a name used for at least eight different conditions that all cause childhood inflammatory arthritis. Like most illnesses, JIA usually develops from a mix of genetic factors (things a child is born with) and environmental factors (things they are exposed to). Some children, however, are more likely to have a strong genetic cause. This is especially true when symptoms begin at an unusually young age, or when a child also shows other unusual immune-system problems. There is a growing group of conditions called Genetic Errors of Immunity (GEI). In these disorders, a change in a single gene disrupts how the immune system works and leads to disease. GEI can cause many kinds of immune problems, ranging from frequent infections to severe autoimmune diseases. Inflammatory arthritis is common in many of these genetic conditions and is not limited to just one type. Because these genetic conditions are so varied, identifying a GEI in a child can be transformative. It can clarify the correct diagnosis, prompt providers to look for hidden or future complications, guide treatment choices, help predict long-term outcomes, and inform family planning for parents and relatives. Despite this, most children with JIA do not receive genetic testing as part of routine care. When testing is done, it is often narrow and incomplete. While a future in which every patient receives broad genetic testing is still some distance away, we already know that some children are far more likely than others to have a genetic cause for their disease. For these children, we have a responsibility to actively search for that cause. A similar situation exists in Inflammatory Bowel Disease (IBD), and for many years specialized programs for Very Early Onset IBD (VEO-IBD) have transformed how young children with IBD are evaluated and treated, particularly by integrating genetic testing into care. This proposal aims to begin the process of extending the same principles and practices used in VEO-IBD to children with JIA by collecting appropriate samples from JIA patients at high risk.