The SONET/SDH network that has grown to be the backbone of most of the modern telecommunications network was originally designed for optical interfaces that used a single wavelength per fiber. As optical component technology has advanced, it has become more economical to transmit multiple SONET/SDH signals over the same fiber using wavelength division multiplexing (WDM) instead of going to a higher rate SONET/SDH signal. Based on experience with the SONET/SDH networks, the ITU-T defined a transport network that was optimized for cost-effective transparent transport of a variety of client signals over WDM networks. The optical transport network (OTN) architecture is specified in ITU-T Rec. G.872 and the frame format and payload mappings are specified in G.709 for carrying SONET/SDH, Ethernet and storage area network (SAN) signals in a much more cost-effective manner than was possible over SONET/SDH networks. This white paper provides a tutorial overview of OTN, with primary emphasis on ITU-T G.709. The white paper also discusses various constraints that influenced the development of G.709, its current status in the network, and some factors that will affect its future.