People with Primary Immune Deficiency (PID) may develop severe, life-threatening infections as a result of inherited defects in the genes that normally instruct blood-forming cells to develop and to fight infections. PID diseases include Severe Combined Immune Deficiency (SCID), leaky SCID, Omenn syndrome (OS), and Reticular Dysgenesis (RD). PIDs may be treated by transplantation of bone marrow stem cells from a healthy person or, in some cases, by enzyme replacement or by gene therapy. Patients with SCID were among the first to receive bone marrow stem cell (also called hematopoietic cells) transplantation (HCT) more than 40 years ago, and HCT is the standard treatment today for this group of diseases. Since PID diseases are rare, there are not enough patients at any single center to determine the full range of causes, natural history, or best methods of treatment. For this research study many PID centers across North America have organized into the Primary Immune Deficiency Treatment Consortium (PIDTC) to pool their experience and study PIDs together. The overall goal of this study is the prospective evaluation of children with SCID and related disorders who are treated under a variety of protocols at participating institutions. The study aims to identify variables contributing to the best outcomes for HCT.