Simplicity defined

One of the most important but most elusive principles of agile development is simplicity. In XP, it's stated as Simple Design, and other contexts often times referred to as KISS (keep it simple, stupid), and closely related to YAGNI (you aren't gonna need it). Simplicity is a key to successful agile development because it's absolutely necessary to support other agile goals and practices:

• short iterations
• refactoring
• collective ownership
• pair programming
• avoid premature optimization
  

In turn, simple design is enabled by test-driven development and refactoring.

The biggest problem with the simplicity principle is that people often disagree on what constitutes "simple", and I've always struggled to come up with a definition of simplicity that was, well, simple. I came across a quote today that I think sums it up pretty well, from Antoine Du Saint-Exupery.

Perfection is not when there is no more to add, but no more to take away.