Wireless communications is， by any measure， the fastest growing segment of the communications industry. As such， it has captured the attention of the media and the imagination of the public. Cellular phones have experienced exponential growth over the last decade， and this growth continues unabated worldwide， with more than a billion worldwide cell phone users projected in the near future. Indeed， cellular phones have become a critical business tool and part of everyday life in most developed countries， and are rapidly supplanting antiquated wireline systems in many developing countries. In addition， wireless local area networks are currently poised to supplement or replace wired networks in many businesses and campuses. Many new applications， including wireless sensor networks， automated highways and factories， smart homes and appliances， and remote telemedicine， are emerging from research ideas to concrete systems. The explosive growth of wireless systems coupled with the proliferation of laptop and palmtop computers indicate a bright future for wireless networks， both as stand-alone systems and as part of the larger networking infrastructure. However， many technical challenges remain in designing robust wireless networks that deliver the performance necessary to support emerging applications. In this introductory chapter we will briefly review the history of wireless networks， from the smoke signals of the Pre-industrial age to the cellular， satellite， and other wireless networks of today. We then discuss the wireless vision in more detail， including the technical challenges that must be overcome to make this vision a reality. We will also describe the current wireless systems in operation today as well as emerging systems and standards. The huge gap between the performance of current systems and the vision for future systems indicates that much research remains to be done to make the wireless vision a reality