Compositional imaging with electron energy loss spectroscopy (EELS) can be performed in both the energy-filtering transmission electron microscope (EFTEM) and in the scanning transmission electron microscope (STEM). Quantitative elemental distributions are obtained from core-edges produced by inner-shell excitations, although more detailed information about chemical bonding and electronic structure is also available from the fine structure associated with valence electron excitations. The fixed-beam EFTEM can provide data from large numbers of pixels very rapidly and offers an advantage for analysis of extended specimen regions containing relatively high atomic concentrations. Acquisition of entire spectra at each pixel in the field-emission STEM (spectrum-imaging technique) provides improved flexibility and accuracy despite the longer recording times. Spectrum-imaging allows post facto data processing with parameters that can be varied after acquisition is completed. In suitably thin specimens, EELS compositional mapping can provide a sensitivity of a few atoms for certain elements.