So…I agree with Swordgleam, but with less eloqeunce.

I keep wanting to run a game in a setting like James Alan Gardner’s excellent book Trapped. All the goverments of the world slowly collapsed for reasons unmentioned, and nearly all advanced technology was lost. Aliens visited earth and made magic and psionics real (sort of). So you’ve got people running around in a world at just slightly above a medieval level of tech: guns work, but not reliably, there’s electricity, but it’s rare, and most people ride horses. Into this mix, add psychics, sorcerors, and some really kickass tech that the aliens dropped off, but only a few people have. It’s a post-apocalyptic setting that’s half fantasy and half sci-fi, and both forces are equally powerful and rare.

I also really like the ideas in Piers Anthony’s Incarnations of Immortality series. Roughly paraphrased, “It’s an open question which would create more havoc: a nuclear bomb being detonated, or a ranking demon of Hell unleashed on earth.” It’s slightly futuristic; technology has advanced, but magic has matched it, so each thing that one can do, the other can do just as well. Magic carpets vie for customers with normal cars, specially made cameras can photograph auras, and there are giant floating shopping malls built on clouds. Purgatory is mostly automated.

Mixing sci-fi and fantasy is great. It just has to be as integral a part of the setting as everything else. If you’re running a medieval campaign and drop in lasers, it’ll be a shock, just as if you’d dropped in dinosaurs or fantasy elements from an entirely different genre; Xanth-style demons and pie trees, for example, would fit very poorly in most D&D games. It’s a matter of tone, too, I think. A gritty game probably isn’t as suited to advanced tech as a high magic game is.

I once had a GM start use in what assumed to be some type of modern day horror setting, only actually abducted by aliens and ending up in a Space 1889 type setting.

As you can guess none of the players were happy with that. The game didn’t match what we expected the game was going to be about. The GM couldn’t (or wouldn’t) understand that is what we were expecting.

That is not to say that on should never surprise players. It if often fun to see what they do when they encounter something they didn’t expect, just as long as they buy off the result as plausible for the game that they are playing in.

It certainly seems that certain “futuristic” or “technological” elements, such as steam tech and early firearms, have been gaining more acceptance in fantasy games.

Now, if the back of the book said clearly that the fantasy world was being turned upside down from a new sci-fi entity, then I would not mind — I bought the book with that in mind. Heck, even a ‘mysterious falling star’ would at least give me a guess that it might have sci-fi elements.

I feel most players would be more okay if they knew something was coming up. When you GM a world of your own design and it has sci-fi elements, the players will most likely be given hints and clues as time goes on (if not overt declaration) that there are sci-fi elements. They’ll warm to it — especially if they see an overall integration and design into the world.

But if you just go from fantasy to sci-fi in the speed of a light switch, it will usually be too jarring. They’ll look you over like the front of the book and read the blurb again by asking you, “What are you thinking?”

That said, a romp through a one-off sci-fi abnormality might be good to spice up the game. Really depends on your players and how well it will be received.

I personally would not care for straight up sci-fi, but some psionics or steam punk is fine to me.