In 1994, I developed an infrastructure, inspired by zApp and the Object Window Library (OWL), for developing an accounting system for Windows. In 2000, I developed another infrastructure, inspired by Struts and WebWorks, for developing another accounting system for the J2EE platform. After coaching and watching the development of both systems, I found that not only did the web edition require more advanced programming skills and prerequisites but its total cost was four times more than the client/server’s. Worst of all, the user experience harkened back to the age of green terminals, though the look, after decorating with proper images and cascading style sheets, was modern and fresh. I started wondering whether these problems were intrinsic to the web or if the programming model was simply inadequate. Looking back at the success of desktop applications in the 1990s, the event-driven, component-based programming model played a big role. Blessed by being easy to learn and develop, this model is the standard and best way to handle interactive and responsive user int- faces. Could this model be applied to web applications? After using ZK to develop several commercial projects, I believe I’ve got the answer––yes! The reasoning behind that answer is what I want to share with you in this book.