Generation, transmission, and processing of constantly expanding data volumes in communications is regulated by the “clock speed” produced by RF oscillators, determining the channel capacity and channel density. An RF oscillator performance defines the extent of capability for clutter rejection and detection of small signals in remote sensing. The quality of the signal produced by an oscillator depends on the energy storage element in the oscillator loop. A generalized Opto-Electronic Oscillator (OEO) is a photonic device that generates stable RF or microwave signals using optical energy storage elements. Such an oscillator can outperform its electronic rival in practically any characteristic. An RF photonic oscillator based on a femtosecond mode-locked laser has demonstrated the highest spectral purity at a microwave frequency. A miniature OEO based on resonant optical four-wave mixing produces the lowest phase noise X-Ka band signals if compared with electronic devices of similar size, weight, and power consumption. An RF photonic oscillator can be tuned over an octave keeping high spectral purity and small size. In addition, the OEOs have low environmental sensitivity. These and other features of OEOs enabling photonic synthesis of spectrally pure RF signals will be reviewed in this presentation. Novel trends in the OEO development also will be elucidated.