This book explores today's wireless technologies which are essential in supporting Federal agency missions crucial to the nation and enabling commercial and non-Federal public safety operations that support economic growth and safeguard lives and property. As changing government missions increase demands for mobility and agility, and private sector uses continue to expand, the spectrum's role as a critical asset intensifies. To ensure that United States' spectrum policy for the 21st Century continues to harness fully the power of the airwaves to meet Federal mission requirements and to enhance prosperity, President George W. Bush established the Spectrum Policy Initiative (Presidential Initiative) in 2003. As this book discusses, the goal is a U.S. spectrum policy for the 21st Century that will foster economic growth, ensure our national and homeland security, maintain U.S. global leadership in communications technology and services and satisfy other vital U.S. needs in areas such as public safety, scientific research, Federal transportation infrastructure, and law enforcement.