The question of how much design to do up-front on a project is an engaging conundrum. Too much design often results in excess complexity and wasted effort. Too little design results in a poor architecture or insufficient system structures which require expensive rework and hurt more in the long run. How can we know the right balance of upfront design work versus emerging design approaches? Alan Shalloway shows how to use design patterns-coupled with the attitude from agile of “don’t build what you don’t need”-to guide your design efforts. The trick is to identify potential design alternatives, analyze how each may affect the system in the future, and then find the simplest approach for isolating those potential affects. Alan describes the essence of emergent design-that is, start with a simple design and let it evolve as the requirements evolve-and demonstrates how to refactor to achieve better designs, which really is quite different from refactoring bad code.