Ajax (Asynchronous JavaScript and XML) encompasses much more than the technologies that make up this catchy acronym. The general term Ajax describes the usage of various Web technologies to transform the sluggish batch submission of traditional Web applications into a highly responsive near desktop-software-like user experience. However, such a dramatic improvement does come with the price of a significant rise in programming complexity, increased network concerns, and new user experience design challenges. For now, let’s avoid most of those details, as is appropriate in an introduction, and begin with an overview of the concepts behind Ajax illustrated by an example. Details of the example will then be presented hinting at future complexity. The chapter then concludes with a brief discussion of the historical rise and potential effects of Ajax upon Web development.