Wireless communications are expected to be the dominant mode of access technology in the next century. Besides voice, a new range of services such as multimedia, high-speed data, etc. are being offered for delivery over wireless networks. Mobility will be seamless, realizing the concept of persons being in contact anywhere, at any time. Two developments are likely to have a substantial impact on the mobile communications systems deployed in the twenty-first century: the adoption of International Mobile Telecommunications-2000 and wireless asynchronous transfer mode (ATM). They are two different but cooperative approaches to providing high-speed wireless access. The limitations of the radio-frequency spectrum and radio channel propagation impose special constraints on the technologies of systems to be deployed. To make future mobile systems globally acceptable, standardization efforts are underway in the International Telecommunications Union and the ATM Forum. This paper reviews the international standardization efforts, the challenges to the technologies imposed by the radio spectrum limitations, radio channel propagation-induced distortions, and possible solutions. Evolution, migration, and architecture issues also are discussed.