The focused ion beam (FIB) tool has been successfully used as both a stand alone analytical instrument and a means to prepare specimens for subsequent analysis by SEM, TEM, SIMS, XPS, and AUGER. In this work, special emphasis is given to TEM specimen preparation by the FIB lift-out technique. The fundamental ion/solid interactions that govern the FIB milling process are examined and discussed with respect to the preparation of electron transparent membranes. TRIM, a Monte Carlo simulation code, is used to physically model variables that influence FIB sputtering behavior. The results of such computer generated models are compared with empirical observations in a number of materials processed with an FEI 611 FIB workstation. The roles of incident ion attack angle, beam current, trench geometry, raster pattern, and target-material-dependent removal rates are considered. These interrelationships are used to explain observed phenomena and predict expected milling behaviors, thus increasing the potential for the FIB to be used more efficiently with reproducible results.