The martensitic (also called displacive or diffusionless) transformation is a classical cooperative phenomenon in solids similar to ferromagnetism. Although the displacement of each atom is not large, the transformation results in a macroscopic change in shape, since all of the atoms move in the same direction in a domain or variant. As a result, unique properties arise, such as the shape-memory effect and superelasticity, whose characteristics are quite distinct from those of normal metals and alloys. Because of these unique properties, shape-memory alloys (SMAs) have been used as new functional materials for applications such as couplings, sensors, actuators, and antennas for cellular phones. In this issue of MRS Bulletin, we present an overview of recent progress in this field. In this introductory article, we discuss fundamental notions, such as the mechanism of the shape-memory effect, the martensitic transformation, and superelasticity, along with examples of applications and other important recent topics not treated in the following articles. It will be shown that progress in the science and technology of shape-memory alloys has been achieved by the side-by-side development of fundamentals and applications.