If we assume polarization multiplexing or a multiple-input multiple-output (MIMO) system with two antennas, 28% forward-error correction and a Nyquist shaping of the data signal (the maximum possible symbol rate that can be transmitted in a given bandwidth), the bandwidth of a 1 Tbit/s signal in a 16-quadrature amplitude modulation (16-QAM) format would be 80 GHz. The processing of such high bandwidths is far beyond the possibilities of todays’ electronics. The requirements may be relaxed by a higher parallelization (more MIMO channels) or a higher spectral efficiency of the modulation (a 1024-QAM only needs 10 GHz). However, these solutions are accompanied by higher hardware costs and especially an increasing power consumption. Photonics may be used to down-convert the signal in the time or frequency domain. Subsequently, the down-converted signals may be processed with low-bandwidth electronics. The basic ideas behind this approach will be discussed in this chapter.