The increasing demand for capacity has driven the telecommunications community to focus on transmission studies with higher data rates and increasing spectral efficiency. However, high data rates and high SE are particularly challenging for transoceanic cable systems due to their extremely long transmission distances. With the advent of digital coherent receivers, the channel data rate has increased from 40Gb/s to 100Gb/s and beyond; the spectral efficiency has increased from 80% (0.8bits/s/Hz) to the most recent 600% (6bits/s/Hz); and the total capacity has increased from 6Tb/s to the most recent 30Tb/s. This chapter gives an overview of the most recent advances in undersea transmission technology since the last edition of Optical Fiber Telecommunications in 2007 focusing on coherent technology and 100Gb/s transmission.