The Gemara (especially the Bavli, not so much the Yerushalmi) is replete with countless references to demons. Nevertheless, Rambam (as well as others such as Ralbag, R. Yaakov b. Abba Mari Anatoli, R. Levi ben Avraham ben Chaim, and probably Meiri and Ibn Ezra) were of the view that there is no such thing. Rambam does not explicitly deny the existence of demons, but it clearly emerges from many different discussions of his.