The technology of combining a number of optical wavelengths and then transmitting the same through a single fibre is called wavelength division multiplexing (WDM). Conceptually, the technology is similar to that of frequency division multiplexing (FDM) used in analogue transmission. Dense wavelength division multiplexing (DWDM) is anew born multiplexing technology in the fibre optic transmission, bringing about a revolution in the bit rate carrying capacity over a single fibre. The emergent of DWDM system is one of the important phenomena in development of optic fibre transmission. This article gives introduction of DWDM technology. There has always been a technological talent to fulfill the constant need to extent the capacity of communication channel and DWDM (Dense Wavelength Division Multiplexing) has dramatically brought about an explosive enlargement of the capacity of fiber network, solving the problem of increasing traffic demand most economically.