Coding Horror, almost two years back, christened a name for the principle variously known as "don't repeat yourself," "once and only once," etc.: Curly's Law. Excellent formulation of the principle. In sending this to my development team, I thought it would be useful to pull this from the abstract to the specific through a few questions:

Lately I've been working with my team to understand and utilize good automated unit testing strategies with NUnit. A code release I was inspecting revealed a couple of good points that seem worth expanding on: the importance of testing each layer, and the need to pay attention to code coverage. This is a rather facile treatment; for more in-depth reasoning and details, I recommend xUnit Test Patterns.