The U.S. government developed atomic energy for peacetime after World War II in the form of radioactive isotopes, produced in a former Manhattan Project reactor and distributed to civilian purchasers. These radioisotopes provided physicians with new tools of diagnosis and therapy and equipped biologists to trace molecular transformations from metabolic pathways to ecosystems. This chapter juxtaposes postwar developments in biochemistry, nuclear medicine, and ecology growing out of this new supply of radioisotopes. In each of these areas, one can see how government policy and infrastructure integral to the Cold War decisively shaped scientific opportunities and knowledge. Routine practices of radiolabeling and radiotracing remained in place long after the positive political valence of radioisotopes dimmed in the 1960s and 1970s, in the wake of the debates over radioactive contamination of the environment from atomic weapons tests and nuclear waste.