Neurath was one of the most conspicuous champions of the ideal of a unified science in the twentieth century. This ideal continues to influence the work of many scientists and some philosophers. However, it is not clear what precise sense should be given to the phrase "unification of science". Different explications of it are possible. In this paper, I examine the notion of theoretical unification, which seems to me the most significant kind of unification. After having provided a formal explication of theoretical unification in terms of the notions of embedding and fundamental theory, I go on to consider what the prospects are for attaining the goal of theoretical unification, both from a synchronic and a diachronic perspective.