A trained police dog, Rex (revealed in the Pilot to be registered as Reginald von Ravenhorst) is the legitimate star of the show. Establishing shots frequently show him demonstrating a new trick - unlatching doors, pushing trolleys, pointing to drugs or corpses - which then turn out to be useful in the course of the episode. He is often used to stalk suspects, often with a GPS-type object attached to him so that the officers can keep track.
Rex was stolen by criminals as a pup, but managed to escape and befriend a boy, where he helped to solve his first case with the boy.
Initially, Rex and Moser share an apartment at Marrokanergasse 18, Wien-Landstraße, however the pair go house-hunting quite early in the series. The house they eventually find is owned by a man who does not want dogs there, however Rex is able to alert him to a gas leak and in gratitude he allows Moser and Rex to stay.
Rex is frequently called upon to resolve difficult situations, including helping a young girl in shock, preventing a woman from committing suicide and helping to get Moser's mobile phone when a crime has been committed. One famous episode features Moser using Rex to resolve a hostage situation by telling him to creep up behind the criminal and "frighten him" (following Rex's earlier success at frightening Stockinger by jumping on him from behind).
There is also a considerable element of humour in Rex's activities. He constantly annoys Stockinger by pulling on his coat and stealing his ham rolls. Later, Höllerer keeps a running score of Böck's success against Rex - not a flattering result for the officer. While not chasing criminals, he often plays pranks on the officers or fails to obey orders to help with the housework.
Rex reacts particularly badly to words like "Frau" (woman) and, later, "Tierarzt" (vet), as he disapproves of his masters' attention to women. During a scene in which Koch claims that Moser "doesn't understand women at all now", Moser tells Koch, "Don't say that word. Every time you say 'woman' he [Rex] runs off with my laundry". Koch asks him what Rex does with it, to which Moser replies, "He washes it". Later, after the departure of the vet, Moser is working undercover and needs to ensure Rex won't greet him. He tells Stockinger, "You only need to say 'vet' to him and he'll stop whatever he's doing", a statement which results in a memorable scene involving Stockinger walking after Rex at a crime scene calling out "Vet! Vet!"
Rex has an uncanny penchant for ham rolls, or "Wurštsemm'ln" in the local dialect. He is introduced to them by Moser, who tells him, "I practically live on these".
Rex has to date been played by three dogs. Until 2000, he was played by Santo vom Haus Zieglmayer, also known as 'Beejay', and was then replaced by Rhett Butler. For the 2008 revival, Rex is played by a dog named Henry.