Science fiction is one of the most interesting genres of film, but it is a very tricky one to define. Author Mark C. Glassy famously wrote in his book, The Biology of Science Fiction Cinema, that “the definition of science fiction is like the definition of pornography: you do not know what it is, but you know it when you see it.”

To create a science fiction movie is to create a whole new world (or, at the very least new rules for the world as we know it). This requires filmmakers with bold imaginations who are not afraid to take risks with their work and make something that is truly out there. But these are also things you could say of works in the fantasy or horror genres; science-fiction is distinct from these in a few ways. The very best science-fiction is at least somewhat grounded in reality: sure, Star Wars is a far cry from a documentary, but the curiosity and drive to explore galaxies far, far away is one of the most basic elements of humanity. As a result, good sci-fi paves a path for questioning the world we live in. The futuristic storylines can often double down as social commentaries, asking us to take thoughtful and sometimes uncomfortable looks at the parts of humanity that could create the worlds we see depicted on the screen.

Although all superhero movies could, in theory, fall into the sci-fi category, there have been enough sequels and prequels and origin stories released in the past few years from both Marvel and DC to warrant their own kind of category, and so we have not considered any superhero movies for this list. What we have considered, though, is 25 of the very best sci-fi movies of all time. Watch all of them and consider yourself a proud nerd.

Related: Best Sci-Fi Movies Streaming on Netflix
Related: All Time Best Sci-Fi Movies