The paper briefly refers to a number of the, by now well known, results of the author?s studies of Software Evolution since they provide a basis and framework for the development of a theory of the phenomenon. He will then summarise his most recent results outlining a proof that every E-type program reflects an unbounded number of assumptions about the application implemented, supported or modelled by the program. He shows that the presence of such assumptions is inevitable and that some of these will become invalid over time as a consequence of changes in the dynamic real world. Some of the finite set of known assumptions reflected in the program will also become invalid. Together the resultant ever extending invalidity will cause the software to require continuing change or to become ever more unsatisfactory or even invalid.