IEEE’s 5G wireless initiative has the goal of serving many more users with much higher transmission speeds. But with the existing cellular bands tightly packed, where does all the required additional network capacity come from? In contrast with the traditional radio-spectrum management view of scarce capacity, where a finite amount of spectrum must be divided up among users, communication theorists see wireless capacity as virtually unlimited. Capacity can be increased indefinitely by going to ever smaller cells and higher frequencies that offer more bandwidth, while greater efficiency can be achieved with advanced signal processing and new spectrum-sharing policies. Among all these approaches, the greatest immediate impact would be achieved by moving to the higher frequencies in the millimeter range—the region of 30 to 300 gigahertz, where bandwidth is available and plentiful.