Published in the April 2003 issue of the Journal on Selected Areas in Communication, this tutorial paper introduces "multiple-input-multiple-output (MIMO) space-time coded wireless systems" and discusses recent progress in their design and performance. Hailed as a breakthrough emerging technology that has profound implications for digital communications, these systems have the potential to resolve current capacity limitations for wireless Internet traffic. The first section of this paper consists of background material and a high-level overview of MIMO systems. The remaining sections go into more of the technical aspects of the technology, describing a general system architecture, MIMO Information Theory, and communication algorithms.