A fictionalist theory is a theory that concerns a particular sphere of discourse, and is meant to encompass three theoretical virtues: a naturalistic ontology; an antirevisionary attitude concerning our discursive practices; and the ability to foster semantic uniformity across discursive spheres. There are compelling reasons to believe that fictionalist theories are superior to their rivals; and, due to the promise that these theories appear to bear, I believe that it is worth inquiring further, and more systematically than it has been done in the past, whether or not they are truly viable. In this thesis, I will contend that they are not. The thesis comprises two parts. Part One contains an argument that calls into question the tenability of fictionalist theories in general. In order to advance this argument, I will discuss three of the extant theories of the ontology of ficta; I will advance a novel proposal thereof, and I will articulate an original solution to a prominent puzzle within the philosophy of music: the audibility problem. Part Two comprises discussions of three distinct fictionalist theories: mathematical fictionalism, modal fictionalism and moral fictionalism. I will argue that these three theories are all objectionable on the very same grounds, and I will thereby suggest that fictionalist theories in general are flawed in a systematic manner.