Impression trays are devices used to carry, confine, and control impression material in the mouth. There are two main types: special trays which are custom-made for individual patients and used for final impressions, and stock trays which come in various pre-formed sizes and shapes and are used for primary impressions. Impression trays must be rigid, dimensionally stable, smooth, and able to support the set impression material when removed from the mouth. They are classified based on their fabrication method, arch, presence of teeth, and materials used.