The incorporation of isotope labels into proteins is extremely useful for the application of nuclear magnetic resonance (NMR), X-ray or neutron-diffraction crystallography, and mass spectrometry (MS) methodologies to investigate the structure and dynamics of proteins. This unit presents methods for incorporating isotopic labels into proteins via expression in E. coli and baculovirus transfected Sf9 insect cells or through cell-free means. The unit also presents methods for introducing isotopic labels by chemical means into synthetic peptides by solid phase peptide synthesis or into isolated proteins by chemical modification of labile protein groups.