Communication occurs when one individual, the sender, produces a signal that alters the behavior of another individual, the receiver. The signal can provide broadcast information about species and individual identity or transmitted information, in which the sender effects a change in the receiver's behavior. To reduce ambiguity, signals have evolved to be conspicuous, redundant, and stereotypic. These features allow communication signals to be produced by senders and perceived and acted upon appropriately by receivers, both conspecific and heterospecific. The modality of the communication signal can be visual, olfactory, or acoustic. Visual signals can be adjusted rapidly for response during interactions between individuals at close or medium range. Examples include displays of relative dominance or submission. Olfactory signals can be used for individual, sex, or group identity at close range during greetings and assessments of individuals. Excretory products can be used for olfactory communication over long distances and for long periods of time. Acoustic signals can be adjusted rapidly for close- and far-range communication. They do not persist in the environment. For dogs, communication is fundamental to maintaining affiliations, reducing competition, and identifying individuals. These factors are critical to the highly developed social behavior of dogs. In an ultimate sense, dogs have been selectively bred for positive interactions with humans; in a proximate sense, many dogs spend their lives in close social association with humans. For these reasons, many of the signals used by dogs in dog-dog communication are also used in dog-human communication. Veterinarians act as receivers for communication signals when greeting dogs as patients. The information obtained is used to assess the state of arousal and probability of future behavior of the dog so that handling of the animal can be facilitated. The goals are to minimize stress and injury, to successfully complete the treatment program, and to promote the health of the animal.