 Part of the structuralistic program is the definition of various intertheoretic relations. Here we will concentrate on the relation(s) of “reduction”, which play an important rôle in the philosophical discourse as well as in the work of the physicists, albeit not under this name. Consider a theory \(T\) which is superseded by a better theory \(T'\). One could use \(T'\) in order to understand some of the successes and failures of \(T\). If there is some systematic way of deriving \(T\) as an approximation within \(T'\), then \(T\) is “reduced” to or by \(T'\). In this case, \(T\) is successful where it is a good approximation to \(T'\) and \(T'\) is successful. On the other hand, in situations where \(T'\) is still successful but \(T\) is a poor approximation to \(T'\), \(T\) will fail. For example, classical mechanics should be obtained as the limiting case of relativistic mechanics for velocities small compared with the velocity of light. This would explain why classical mechanics was, and is still, successfully applied in the case of small velocities but fails for large (relative) velocities. 