Studies of DNA replication in chromosomes first clearly revealed the compartments of the cell cycle and provided a means for measuring the parameters of the S-phase in the cell cycle. Radioisotopic labeling, especially with [3H]-thymidine, provided a specific label easily detected by autoradiography, as well as by liquid scintillation counting for biochemical studies. The research utilizing these tools soon centered on what seemed to be two separate areas. One that occupied our attention for many years deals with the structural properties of chromosomes and the exchanges and aberrations that could be revealed. Most of these problems are now understood, at least in broad outline. The discovery of fluorescence and staining procedures to distinguish bromodeoxyuridine (BrdU) labeled segments from those with thymidine in chromosomal DNA stimulated the structural studies, particularly in the last 10–12 years. In this review I will spend little time discussing this area; for an extensive review of some aspects see Tice and Hollaender (1984).