Arthritis is a chronic, disabling disease of childhood. Many children with arthritis will continue to have arthritis as adults. New medications called biologic therapies are now available to treat arthritis in children. They target key biologic molecules that cause inflammation. While they are effective, they may have serious side effects including infections and cancer. They are also expensive. If we know the biological characteristics of each child’s arthritis, then we could select the most effective biologic therapy for that child. Thus, we would improve the health of each child, be able to prevent medication side effects and reduce the health care burden for patients, families and society. One of the many advantages of studying diseases in children is that we will learn about the underlying biology without the complicating influences of aging, environmental exposures and lifestyle choices that affect adults. This research will promote a better understanding of the biology of arthritis which will lead to better therapies and eventually a cure for arthritis.