Man against machine is a very common science fiction theme. It centers around a truth that we can see happening all around us; humans becoming dependent on technology. The Terminator films and many Ray Bradbury stories deal with this theme, commonly using it as a warning for the future.

People obtaining god-like powers through knowledge or technology is another very common them in science fiction. It can be seen in the Dune book series and the Stargate television series very prominently.

Time travel is a very popular theme. It was used in the H.G. Wells story "The Time Machine" and continues to be used to this day. Quantum Leap, Star Trek and Doctor Who all use time travel prominently. The importance of how the world could be forever changed just by one small action is usually the most important aspect of time travel when it is used in sci fi.

Space exploration or exploration of any kind are essential in science fiction. Nearly every science fiction plot centers around the idea of exploring--"going boldly" as Star Trek puts it--and it is an very typical theme to encounter as well.

Over the last few years, the most common sci-fi theme is robots and artificial intelligence, and aliens. It is likely that these types of sci-fi have gained even more popularity than normal because today’s technology tends to be leaning towards actually attaining an artificial intelligence computer or robot. Films like Terminator have had several different sequels because the movie preached the dangers of allowing A.I. to go unchecked.

As we send more and more probes into outer space the likelihood that we will eventually find proof that there is life in other galaxies or planets grows. This is why we have seen a lot more alien invasion movies like Skyline coming out in the last few years. Science fiction that doesn’t seem to unrealistic is always the most popular.