Imaging and therapy of cancer using radionuclide-labelled compounds is surveyed. The current state of knowledge concerning the cancerous process, the subtle differences between the cancer cell and the normal cell, and the biological basis of the response of cells to low-level and internal targeted radiation are described. There is movement from the surface of the cancer cell with antigen, receptor and transport protein detection, to the cytoplasm with the monitoring of signal transduction and gene therapy, to the nucleus with oligonucleotides antisense to oncogenes. Nuclear medicine's shift from physics to biology is from the nucleus of the atom to the nucleus of the cell.