100 gigabit Ethernet (100 GbE) is widely envisioned as the next-generation Ethernet, driven by continued capacity growth in provider networks, new emerging high-bandwidth applications, and an evolutionary trend from TDM to packet transport, particularly in the metro space. We will describe some key application areas where 100 GbE will probably be employed first. We further discuss possible implementations on the physical layer (PHY) and argue for the need of an efficient framer and against a medium access controller (MAC). Particularly for MANs and WANs, the most prominent candidate for the required framing will be based on the optical transport network (OTN, ITU-T G.872/G.709). 100 GbE will be an IEEE 802.3 standard and will hence only define the access control of and the transport on the physical sublayer. However, the challenges for realizing 100 GbE are related to Ethernetswitchingand not to Ethernettransport.