The 40/100 Gbps solutions are finally mature for WDM transmission on legacy system and fiber infrastructure of incumbent carriers, which want to upgrade their core and metropolitan transport networks beyond 10 Gbps. One technology, based on the combination of PM-QPSK modulation format, coherent detection and digital signal processing, has solved the dilemma of ultra high bit rate transmission over PMD-limited fiber infrastructure, which has postponed during 10 years 40/100 Gbps deployments. But already, in the R&D laboratories, people start to prepare the next generation of long-distance WDM systems operating at 400 Gbps and 1 Tbps per wavelength. The challenges of 40/100 Gbps deployments on existing system and fiber infrastructure are discussed here, while the most efficient economical conditions for their introduction are determined. The different options for 400 Gbps and 1 Tbps WDM transmissions are compared, while the technical requirements which would allow transporting 50 Tbps overall capacity on more than 1500 km are introduced. The potential consequences of this unbelievable increase of the data rate over the operation of metropolitan and core transport networks are also given, whereas solutions to manage them while increasing the flexibility of these networks are proposed.