Clarity via Isolated Tests

Several Destroy All Software screencasts have touched on isolated testing, but never addressed it directly for its own sake. That's the topic of this screencast: why do we care about isolating the class under test from other classes? We'll look at an actual example I ran into: I started TDDing a class, letting it integrate with some other simple classes. After realizing that the test was becoming a mess, I deleted it, rewrote it in an isolated way, and it was far more readable. We'll retrace those steps to see the dramatic difference isolated testing made.