Microcellular polymers, as a group of foamed materials invented in 1980s have got a great development in 1990s due to the application of supercritical CO\-2 in their processing. They are usually called as \!novel materials for 21 Century\. In this review, the preparation, structure and properties of microcellular polymers, theoretical progress are summarized. The advantages of using supercritical CO\-2 in making microcellular polymers are emphasized. A detail description of basic methods, such as batch meth...