This study is not a treatment protocol and no experimental treatments are involved. Study participants may be seen as needed for clinical, translational and basic research studies, or as medically indicated. Subjects will receive their general medical care outside the NIH and will be seen at our clinic or at CNHS with varying frequency. Subjects may be seen for multiple visits. Subjects may be asked to return for additional testing as needed. Clinical care for patients with sickle cell disease will be provided as appropriate through the Sickle Cell Clinic and the inpatient clinical center.