Most of Asimov's robot short stories, which he began to write in 1939, are set in the first age of positronic robotics and space exploration. The unique feature of Asimov's robots are the Three Laws of Robotics, hardwired in a robot's positronic brain, with which all robots in his fiction must comply, and which ensure that the robot does not turn against its creators.
The stories were not initially conceived as a set, but rather all feature his positronic robots—indeed, there are some inconsistencies among them, especially between the short stories and the novels. They all share a theme of the interaction of humans, robots, and morality. Some of the short stories found in The Complete Robot and other anthologies appear not to be set in the same universe as the Foundation Universe. "Victory Unintentional" has positronic robots obeying the Three Laws, but also a non-human civilization on Jupiter. "Let's Get Together" features humanoid robots, but from a different future (where the Cold War is still in progress), and with no mention of the Three Laws. The multiple series offers a sense of completeness, because all of its works are interconnected in some way.