Semiconductor lasers emitting in mid-infrared (IR) range, 2-5 mu m, have many important applications in semiconductor industries, military, environmental protection, telecommunications, molecular spectroscopy, biomedical surgery and researches. Different designs of the reactive regions in mid-IR laser structures have been investigated for achieving high performance devices. In this article, semiconductor mid-IR lasers with double heterostructure, quantum well, quantum cascade, quantum wire, quantum dash and quantum dot active regions have been reviewed. The performance of the lasers with these different active regions and the development of the newly emerging III-V-N materials for mid-IR applications have been discussed in details. (c) 2007 Elsevier Ltd. All rights reserved.