Despite the important work of previous sickle cell disease (SCD) cohort studies, there remain many understudied areas that require investigation. An important knowledge deficit is the slow but progressive process of chronic end-organ dysfunction. The majority of organ dysfunction becomes apparent in the young adult years, but comprehensive assessment of adults and understanding of predictors of adulthood organ dysfunction are insufficient. Similarly, the role of disease-modifying therapies, such as hydroxyurea, in preventing organ dysfunction later in life is not clear. Extended follow-up of patients through the transition into adulthood is imperative to understand the long-term implications of pediatric sickle cell care. This observational study will collect data in a systematic fashion at participants' regular clinic visits (in-person or remote) to answer the objectives described below. In addition to primary study objectives, SCCRIP participants will be eligible to participate in a sub-study, which will investigate genetically determined responses to Hydroxyurea (HU) via a pharmacokinetic study (PK). This one time study will involve blood collection at timed intervals proceeding a dose of HU. Defining the basis for this inter-individual variability will allow the identification of poor HU responders prior to initiation of therapy and the seeking of alternative treatments which seek to optimize disease treatment by accounting for individual variability in genes, environment, and lifestyle.