A viewing panel for location within a door or like structure comprises a frame within which are retained fixed and slidable apertured plates located between fixed sheets of transparent material. The slidable plate is movable by an arm secured to the plate which protrudes through an elongate slot formed in one of the transparent sheets between a first position in which the or each of its apertures is aligned with that or each of the apertures of the fixed metal plate and a second position in which the or each aperture is aligned with one or more land sections of the fixed plate. The land sections of one or both metal plates carry strips of intumescent material which intumesce in the presence of excessive heat completely to fill the panel interior.