The Doctor started his service on the Starship Voyager as an Emergency Medical Holographic Program, (or EMH) built into the starship's sickbays as a stop-gap measure for use if the ship's doctor should be temporarily unable to perform his duties. In the first episode, Voyager's human doctor was killed, and the EMH was called into duty.
Over the course of Voyager's seven seasons, the Doctor's program evolved to become more lifelike, with emotions and ambitions, and developed meaningful and complex relationships with many members of the ship's crew. The Doctor also developed talents as a playwright and artist, and became a connaisseur of the opera. A recurring theme in the series was the set of ethical questions surrounding an artificial, but apparently sentient being. The Voyager crew overcame their initial attitudes towards his artificial form of life, eventually treating the Doctor as a full equal. However, both Starfleet and beings encountered by the Voyager crew did not always accept the Doctor as a sentient being with all the rights afforded to a living person.
The Doctor also acquired a "mobile emitter" which allowed him to move freely, unbound by fixed holographic projectors. He has been decorated for valor in combat, and has saved the ship many times from disaster.
One recurring theme in the Doctor's life was his lack of a name. Starfleet did not give him a name, and for a long time the doctor maintained that he did not want to have a name. Later, over the years, he adopted such names as "Schmullus", "Doctor Schweitzer", and others, before finally, in one potential future, settling on "Joe".
The Doctor is played by Robert Picardo.