September 2004 1527-3342/04/$20.00©2004 IEEE M etamaterials are artificial structures that can be designed to exhibit specific electromagnetic properties not commonly found in nature. Recently, metamaterials with simultaneously negative permittivity (ε) and permeability (μ), more commonly referred to as left-handed (LH) materials, have received substantial attention in the scientific and engineering communities. Science magazine even named LH materials (LHMs) as one of the top ten scientific breakthroughs of 2003 . The unique properties of LHMs have allowed novel applications, concepts, and devices to be developed. In this article, the fundamental electromagnetic properties of LHMs and the physical realization of these materials are reviewed based on a general transmission line (TL) approach. The general TL approach provides insight into the physical phenomena of LHMs and provides an efficient design tool for LH applications. LHMs are considered to be a more general model of composite right/left hand (CRLH) structures, which also include right-handed (RH) effects that occur naturally in practical LHMs. Characterization, design, and implementation of one-dimensional (1D) and two-dimensional (2-D) CRLH TLs are examined. In addition, novel microwave devices based on CRLH TLs and their applications are presented.