|While WDM technology brings huge transmission capacity potential to a single fiber, the capacity requirement of a single traffic request might be far less than the capacity of a single wavelength running on a fiber. Traffic grooming tries to address this capacity mismatch problem by packing low-rate traffic requests into high-rate wavelength channels (lightpaths) in a cost-effective way that either resources requirement is minimized when traffic requests are given or network throughput is maximized when the amount of network resources is given. Because SONET is the most widely deployed optical network architecture, traffic grooming in SONET/WDM ring networks has been a research focus for several years. With the development of optical switching cross-connects, IP/WDM seems to be a promising architecture to meet the tremendous traffic increase of Internet. Since Internet has a mesh topology, traffic grooming in mesh networks also evokes great interest currently. This presentation gives an introduction to the traffic grooming problem in SONET/WDM ring networks and WDM mesh networks, including issues such as concepts and benefits of efficient traffic grooming, key technologies enabling grooming, mathematical formulation of the traffic grooming problem, computational complexity of traffic grooming problem, grooming heuristics and traffic models.