What is the slowest group velocity of light?——an introduction to developments of researchon ultraslow group velocity

The fundamental concepts of the phase velocity and group velocity of light, the dispersion property of a medium and electromagnetically induced transparency are discussed. The developments of research on ultraslow light are reviewed and two experiments, by Hau in 1999 and Phillips in 2001, are described in detail. To conclude we discuss the significance of such research for science and applications.