Simulation has been used to better understand the process parameters which affect focus monitor performance. Full resist process simulations were done using PROLITH/2. Exposure dose, partial coherence and focus monitor linewidth were varied, assuming an aberration-free lens. The focus monitor result was in good agreement with simulations of two traditional focus test approaches. Simulations were also done with optics having significant third order spherical aberration. In this case, the results of the two traditional focus methods differed with each other, and the focus monitor gave another significantly different result. The focal plane of the aberrated lens depends on what pattern is being printed. Determining the crossing point of focus monitor calibration curves with different partial coherence may allow the lithographic measurement of spherical aberration. This paper also outlines recommendations for the practical use of the focus monitor, along with two examples. The first example illustrates a lens heating problem when using a stepper at a non-standard (sigma) value. The second example demonstrates a focus problem at the edge of the wafer caused by a non-flat chuck.