Abstract: A safety-type viewing window assembly for protective housings and protective doors of material-processing machines, such as grinding or metal-cutting machines. The assembly includes a fixed, transparent glass pane coupled along its periphery to a fixed, transparent, puncture-proof or unbreakable protective pane made, for instance of polycarbonate. The glass pane is on the side of the assembly toward the machine and the protective pane is on the side toward the user. The glass pane has a circular opening which is covered by a circular, transparent, rotatable window made of transparent glass which is coupled, on its side facing toward the protective pane, to a rotary drive mechanism. The diameter of the rotatable window is slightly less than the diameter of the opening in the glass pane. The rotatable window is disposed inside the opening in the glass pane, and the gap between the periphery of the rotatable window and the opening in the glass pane is sealed.