This study examines the spin coating process both experimentally and from a fundamental point of view. The analysis has produced a model and a quantitative relationship between spin coating thickness and pertinent material and process variables. The model predicts that the simplest and most reproducible results occur in a region which is independent of thickness and time and is characterized by low steady state radial flow on the substrate. The model also includes a time-dependent term, shown to be important for the region of high radial flowfluids of relatively low viscosity and process conditions of relatively high speed and/or long times. Experimentally, Newtonian-like polyamide in iso-amyl alcohol solutions was examined on large rotating substrates, and measurements showed excellent correlation with the model. The sensitivity of the spin coating process to substrate size and shape is also reported and a comparison with other data in the literature, particularly on photoresist solutions, is presented. The potential applicability of the model to non-ideal fluids is also developed.