Identification, monitoring, and analysis of genetically modified cells in the peripheral blood are an important component of the clinical follow-up of patients treated by hematopoietic cell gene therapy. Analysis of gene-marked peripheral blood cells provides crucial information on gene transfer efficiency as well as on the nature and characteristics of the genetically modified cells, and may provide early evidence of the occurrence of potentially detrimental side effects. T lymphocytes are a convenient target for this type of analysis, due to their abundance and their relatively long life span in vivo. Tracking of gene-marked T cells is based on relatively simple, FACS- and PCR-based techniques, which may be applied to monitoring genetically modified T cells as well as T cells derived from transplanted, genetically modified hematopoietic stem cells. This chapter provides a description of these techniques and clues to their rational use in a clinical setting.