Many laboratory rodents emit ultrasonic vocalizations. The purpose of this review is to highlight the types and functions of ultrasonic vocalizations emitted by laboratory rats and mice. Rats emit 3 types of ultrasonic vocalizations, depending on the animal's age, its environmental conditions, and its affective state. Rat pups emit a 40-kHz vocalization when they are separated from their mothers. Adult rats emit a 22-kHz vocalization in anticipation of inescapable aversive stimuli. These two types of vocalizations reflect a negative affective state of the animal. Rats produce a 50-kHz vocalization under nonaversive conditions, and these vocalizations reflect a positive affective state of the animal. Adult mice produce several different types of ultrasonic calls that can be classified as different syllables. Mice produce ultrasonic vocalizations during nonaggressive interactions, particularly during mating behaviors, but these vocalizations are not indicators of negative or positive affect. Therefore, the function of ultrasonic vocalizations in adult mice is likely only to facilitate or inhibit social interactions. Understanding the types and functions of ultrasonic vocalizations emitted by laboratory rodents may enable researchers and animal care personnel to use vocalizations as an indicator of an animal's behavior and affect.