This volume is a collection of articles about dynamic geometry: active, exploratory geometry carried out with interactive computer software. This software has had a profound effect on classroom teaching wherever it has been introduced. Unconstrained parts of the configurations are moveable - they can literally be grabbed with a cursor (using a mouse) and be dragged or stretched - and as they move, all other objects in the configuration automatically self-adjust, preserving all dependent relationships and constraints. The software has also become an indispensable research tool for mathematicians and scientists. This book gives many examples of the ways in which it can be used, and some of the effects it can have. It raises various questions for teaching and research. Some articles address the basic question, 'What is dynamic geometry good for?' as they discuss: accuracy of construction, visualization, exploration and discovery, motivating proof, transformations, tracing loci, simulation, and creating microworlds.