Every component in a system depends to a greater or lesser extent, on the implementation of its peers for its own continued smooth running. By definition, then, development breaks systems. As you improve your classes and packages, you must remember to amend any code that works with them. For some changes, this can create a ripple effect, affecting components far away from the code you originally changed. Eagle-eyed vigilance and an encyclopedic knowledge of a system’s dependencies can help to address this problem. Of course, while these are excellent virtues, systems soon grow too complex for every unwanted effect to be easily predicted, not least because systems often combine the work of many developers. To address this problem, it is a good idea to test every component regularly. This, of course, is a repetitive and complex task, and as such, it lends itself well to automation.