AJAX DOES A LOT FOR WEB USABILITY, HAS ALREADY DELIVERED SOME STUNNING APPLICATIONS, AND IS clearly the "in" thing at this time. But it's no magic bullet. Careful design is always required, and it must be tailored to the technology at hand. By monitoring the state of Ajax applications, we continue to learn about what works and what doesn't, and about how developers are succeeding in their design trade-offs. This chapter explains these lessons at a high level and introduces the patterns, which discuss them in depth.