If you like sci-fi (and who doesn't?), there are plenty of Halloween costume choices. Some of the more common ones are still popular, and there are costumes that come and go with movies and books that hold popularity for a while and then fade out. Star Trek and Star Wars - almost any character for either franchise - have been popular since they were first created, and they'll remain favorites for a long time.
Costumes for the Younger Crowd
If you're looking for a costume for a younger child, you should consider something that's relatively simple, like a robot. That kind of costume is a favorite of many people, and it's easy to move around in. If you buy a costume, make sure it fits properly. If you decide to make your own, it's best to be sure it's comfortable and that your child can see out of it easily. If it's too hard to maneuver, it's not a good choice for a younger kid who may not be able to walk around that well in a cumbersome outfit.
Older Children and Adult Sci-Fi Costumes
If you have older children, or you're looking for a costume for yourself, some favorites include the aforementioned Star Wars and Star Trek characters, as well as costumes from Sci-Fi animated shows such as Futurama. You can also be inventive with costumes and choose something more otherworldly, such as an alien.
There are so many chances to be creative with costumes, because sci-fi is all about things that are far beyond what people would recognize or expect today. With that in mind, you can choose almost anything that embodies that spirit of uniqueness and that shows off your personal understanding of what sci-fi really means to you.
There are tons of Sci-Fi themed infant Halloween costumes, however, most are found online than in the store.