The measurement of resist image size using a finely focused ion beam (FIB) is described. Comparisons of FIB and scanning electron microscope (SEM) measurements of resist features are presented. The short penetration range of an ion relative to an electron is shown to offer fundamental advantages for critical dimension metrology. Ion beams can induce less dimensional change (damage) than SEMs during the measurement process, potentially yielding improved measurement precision. Resist cross sectioning using FIB is demonstrated, and the factors contributing to quality of the cross section are presented. An H2O gas assisted etching process for polymer resists has been developed and is shown to significantly improve the quality of resist cross sections.