An impression tray (10) for making impressions of the upper and lower teeth. The tray includes a mouthpiece (13) and a handle (12). The mouthpiece (13) includes an outer wall (14), inner wall (16) and a base (18) connected between the walls (14, 16) at a bottom thereof. The walls (14, 16) each have vertical slits (54, 68) spaced along their length. Each slit (54, 68) has a shoulder (60, 73) therein providing an interlock for the impression material displaced therethrough. The base (18) includes ribs (88) on its top, each rib (88) extending between the walls (14, 16) and alternating with the slits (54, 68) on the walls. Between each adjacent pair of ribs (88) are two holes (90) extending through the base (18). The base (18) is also attached, on its bottom, to a platform structure. The platform structure provides a flat surface for the tray and forms reservoirs (102) around the base holes (90). The combination of the ribs (88) and the platform structure provides a rigid structure, thereby producing an impression with a minimal possibility of deformation.