We live in a three-dimensional world, actually four dimensions if time is included. In the short history of visualization research, most graphical display methods have required that data be plotted on sheets of paper, but computers have evolved to the point where this is no longer necessary. Now we can create the illusion of 3D space behind the monitor screen, changing over time if we desire. The big question is why we should do this. There are clear advantages to conventional 2D techniques such as the bar chart and the scatter plot. Designers already know how to draw diagrams and represent data effectively in two dimensions, and the results can easily be included in books and reports. Of course, one compelling reason for an interest in 3D space perception is the explosive advance in 3D computer graphics. It has recently become cheap to display data in an interactive 3D virtual space and so people are doing it, often for the wrong reasons. It is inevitable that there is now much ill-conceived 3D design, just as the advent of desktop publishing brought much poor use of typography and the advent of cheap color brought much ineffective use of color. Through an understanding of space perception, we hope to reduce the amount of poor 3D design and clarify those instances in which 3D representation is really useful.