Bone marrow engraftment studies are used to evaluate the level of donor versus recipient cells in post-transplant peripheral blood or bone marrow specimens. Unique fingerprints identified from recipient and donor DNA are used to determine the proportion of each contained within the total DNA extracted from the post-transplant specimen. These percentages correspond to relative amounts of donor and recipient cells in the specimen. Engraftment studies are sequentially performed on transplant patients to closely monitor the levels of donor and recipient cells so that appropriate therapeutic intervention can proceed if and when needed. This unit will describe the use of fluorescent PCR for amplification of genomic short tandem repeats (STR). STR analysis is now considered the gold standard for engraftment studies and provides a quick and accurate assessment of the contribution of both donor and/or recipient hematopoietic cells in post-transplantation specimens.