This part aims at giving keys to understand why erbium-doped fiber amplifiers (edfa) have revolutionized signal transmission techniques and how they can complement with Raman amplification in next generation systems. A first part is dedicated to physics basics ofedfa followed by the description of the system requirements for both terrestrial and submarine applications. Their related characteristics and second-order effects seen as possible limitations in systems will be reviewed. Future developments based on cladding-pumping or operation in the L-band and benefit of using Raman distributed amplification will be analyzed. Finally, amplification solutions dedicated to operation in new bands will be discussed as well.