Coherent DWDM Technologies: Network bandwidth demand is growing at a staggering rate, estimated at approximately 40% year-over-year, largely driven by cloud, mobile, and video. Techniques to increase optical bandwidth cost-efficiently through WDM have begun to get more complex and therefore more technically challenging to implement. This paper reviews the progress and techniques used to increase optical channel capacity from 2.5G to 100G and beyond, with a particular focus on coherent transmission technologies, and stresses the importance of practical and robust implementations that deliver cost-effective and highly reliable optical transport.