This chapter describes the most important anatomical and physiological features of the laboratory rat. The body of a normal, healthy Norway rat is long and slender. The tail is hairless and may be as long as 85% of the total body. The tail is proportionately longer in females than in males. Growth rates and maturation times for rats varies by strain. Body weight varies greatly, not only with age but with stock, strain, and source of the rat. Both fore- and hindlimbs have five digits. The first digit is greatly reduced on the forelimb and has a flattened nail unlike the more rounded nails of the other digits. Typical pads are present and likely provide cushion against forces placed on the feet during walking and rest. The forelimb has five apical pads, three interdigital pads, and two basal pads. The skeleton of the rat is typical for mammals. The rat has been used extensively as a model for studying the effect of space flight and microgravity on the skeletal system.