Molecular biology has brought about a new dimension to our understanding and analysis of human cancer. Diagnosis and prognosis of cancer have increasingly benefited from the use of molecular markers and molecular approaches. Although the description of circulating tumor DNA in the blood has a long history, the clinical implications of its presence require a large body of well-controlled and stringently performed large research and clinical tests. This chapter has provided an overview of the essential elements for the preparation and analysis of circulating DNA in the plasma or serum. We hope that the basic approaches and tips summarized here will serve as a foundation for the rigorous and comprehensive molecular analysis of circulating nucleic acids in the years to come.