There's an irony about agile development. There is no hard evidence that it produces better software, faster. And formal adoption rates, admittedly hard to measure, don't reach the 20 percent mark. Yet the ideas that underpin agile development--defining requirements incrementally, writing software in short stints, seeking customer feedback, testing code as it's written, frequent builds--have caught on like wildfire. They are widely accepted as sound development practices, even among teams that have not formally adopted them.