DNA and RNA fractions have been isolated from the whole blood, serum, plasma, the surface of blood cells, and urine of both healthy individuals and patients. The ability to isolate, quantify, and analyze these molecules has led to the identification of specific nucleic acid fragments related to particular disorders such as diabetes, cancer, myocardial infarction, and stroke, threby permitting their early diagnosis. Currently, a number of methods for isolating the nucleic acids are employed and although a start has been made to compare the efficiencies of these methods, there is still a way to go before there are precise protocols for nucleic acid extraction. The older chemical methods of extraction still outperform some of the available kits. Some progress is being made to determine the origin of the circulating nucleic acids, although there are still many questions to be answered, including whether the source is through the spontaneous release of newly synthesized nucleic acid or whether it just derived from necrotic and apoptotic cells. In addition, it can be demonstrated that the nucleic acids can enter cells and exhibit a biological activity in the recipient cells. Hence, the question remains: Are the circulating nucleic acids freely entering tissues and cells from the blood and inducing changes in those tissues and cells? Further work is needed to elucidate these areas, and the various protocols must be standardized if the new methodology is to be widely and accurately applied in the diagnosis of disease and the monitoring of therapy. This chapter summarizes the work reported in this volume.