RoboCup is an increasingly successful attempt to promote the full integration of AI and robotics research. The most prominent feature of RoboCup is that it provides the researchers with the opportunity to demonstrate their research results as a form of competition in a dynamically changing hostile environment, defined as the international standard game definition, which the gamut of intelligent robotics research issues are naturally involved. This article describes what we have learned from the past RoboCup activities, mainly the first and the second RoboCups, and overview the future perspectives of RoboCup in the next century. First, the issue on what and why RoboCup is addressed, and a wide range of research issues are explained. Next, the current leagues are introduced and the research achievements are reviewed from a viewpoint of system architecture. Some of these achievements are included in this special issue. Finally, prospects for future activities are discussed.