Lehman's laws of software evolution

In 1980 Lehman and Belady came out with two laws explaining why software evolution could be the longest of the life cycle processes. These can be summarized as below:

1. Law of continuing change: To continue being useful, a system must undergo changes.
2. Law of increased complexity: The program structure detiorates as changes are introduced into it over time. Eventually, the complexity rises to an extent where it is cost effective to write a new program than to try to maintain it.