Today a 40-Gbit/s data rate is agreed by major optical telecommunication players as the next step in the network evolution, with an actual deployment foreseen in the 2007-2008 timeframe. R&D activities on technologies for 40-Gbit/s products are currently active but the path to 40-Gbit/s transponders is not yet fully settled. In this paper, we review the different component technologies currently considered for the actual development and the implementation of future 40-Gbit/s transponders. Dedicated paragraphs are devoted to electronic ICs and electrooptical devices, along with considerations on the technical solutions ensuring suitable interconnections or integration of the different components. Such advanced transponders should be compliant with the requirements of the different segments of the optical transport market. Solutions derived from choices made at lower data rates are projected for the shortest transmission paths, based on conventional nonreturn to zero modulation. In the peculiar case of long-haul transmission, signal distortion resulting from fiber propagation impairments calls for the generation of alternative modulation formats at the transmitter side and the potential need for electronic processing at the receiver side. This obviously has a clear impact on both the transponder design and the individual components features. Finally, recent advances in the field of innovative "all-optical" transponders implementing optical regeneration are also reported