Disclosed is a process monitoring method, and an associated metrology apparatus. The method comprises: comparing measured target response spectral sequence data relating to the measurement response of actual targets to equivalent reference target response sequence data relating to a measurement response of the targets as designed; and performing a process monitoring action based on the comparison of said measured target response sequence data and reference target response sequence data. The method may also comprise determining stack parameters from the measured target response spectral sequence data and reference target response spectral sequence data.