Advances in thin layer chromatography (TLC), including smaller more uniform particles, use of a scanning spectrophotometer (densitometer), and sample application devices, led to the development of the High Performance Thin Layer Chromatography (HPTLC) technique. HPTLC allows quantitative as well as qualitative results of much smaller amounts. in some cases down to the picogram level. With these advancements, the limiting factor in detection of smaller concentrations has become the plate itself, and more specifically the preparation of the absorbent and binder and the layering process. This research evaluated HPTLC plates from several manufacturers for significant differences between manufacturers and between plates of each manufacturer. Several concentrations of three drugs of abuse were applied, developed, and quantitated. Both Rf and peak area were statistically evaluated to look for any effect of manufacturer, specific plate for that manufacturer, specific drug, concentration, and/or cross nested effects. Significant differences were found between manufacturers for both Rf and peak area with E. Merck and Baker plates having the best overall results. All manufacturers were found to have some plates with obvious visual surface defects that were not suitable for use. The major source of variation for all manufacturers was the plate to plate variation rather than track to track deviations on any given plate. / Master of Science