 Scientific theories seem to have an expiry date. If we look at the history of science, a number of theories that once were dominant and widely accepted are currently taught in history of science courses. Will this be the fate of current scientific theories? Is there a pattern of radical theory-change as science grows? Are theories abandoned en bloc? Or are there patterns of retention in theory-change? That is, are some parts of theories more likely to survive than other parts? And what are the implications of all this for the scientific image of the world?