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The question of whether the glass transition temperature in thin polymer films depends on the film thickness or not has given rise to heated debate for almost two decades now. One of the most puzzling findings is the seemingly universal thickness independence of the dielectric α-relaxation observed for supported films. It is puzzling not only in view of the fact that other techniques or other geometries sometimes showed a significant shift of as a function of film thickness, but more so, because computer simulations for all types of polymer film models revealed changes in the structure and dynamics close to a hard surface or a free surface. Our results suggest to explain this apparent contradiction by the fact that only within 1–2 nm from the wall the density changes are sufficiently large to alter the dynamics. Additionally, the wall desorption kinetics, which introduces a new energy scale (at least for simple van der Waals attraction), is enslaved to the α-relaxation at low temperatures.