A tutorial on opto-electronic clock regeneration at very high bit rates beyond reach with purely electronic solutions is given. Emphasis is placed on sum frequency generation in a nonlinear material such as LiNbO3. We first provide a basic introduction to CR (clock recovery) and a PLL (phase-locked loop); two examples are considered, an input signal frequency step and a slow input signal frequency. Next we discuss opto-electronic clock recovery based on an OPLL (opto-electronic PLL). The OPLL contains a phase comparator consisting of a planar LiNbO3 waveguide, a lowpass filter, a VCO (voltage controlled oscillator) and a local oscillator laser. The error signal from the comparator determined by the difference in electrical phase between the signal and the VCO controls the VCO. The VCO has two outputs; one that modulates the local oscillator laser and another that triggers a decision circuit that samples the output from the OPLL. The VCO is continuously adjusted by the OPLL so that it will ensure sampling of the signal in the optimal moments.