Despite the advances in medical management of congenital and acquired cardiac disease, heart transplant remains the only curative option for certain patients. Transplant physicians aim to maintain a fine balance between too much and too little immunosuppression, so as to prevent complications such as infections, malignant growths, and toxic effects of drugs on one hand and acute or chronic rejection of the graft on the other hand. The ImmuKnow assay (by Cylex, recently acquired by Viracor-IBT Laboratories, Inc) was first introduced in 2002 by the Food and Drug Administration for detecting cell-mediated global immunity, thus providing an additional tool to help identify patients at risk for infection and rejection. All studies done to date are reviewed to examine the use of ImmuKnow in heart transplant recipients, both adults and children. Advantages and disadvantages are described, as well as areas in need of further investigation and improvement.