Coarse wavelength-division multiplexing (CWDM) is an ideal solution to the tradeoff between the cost and the capacity. Compared with DWDM, CWDM system deploys uncooled distributed feedback (DFB) lasers and wideband optical filters. These technologies provide several advantages to CWDM systems such as lower power dissipation, smaller size and less cost. For these merits mentioned above, the CWDM system will be widely used in the metro and access networks in the future. The CWDM system is scalable, but the scalability is limit. So, in some metro networks, it can¡¯t meet the increasing needs of the capacity. To solve this problem, there are a few of solutions proposed now, such as the TDM techniques and the combination with the DWDM that is applied in C band. In this paper, firstly, we will introduce the techniques of the CWDM, and explain the reasons of the less cost, and then the solutions to the scalability are presented in detail. At last we introduce the applications in the metro and access networks at present.