After a review of the fundamental and technological factors which are limiting the achievable spectral purity of integrated oscillators, this chapter proposes circuit solutions to break the current phase noise (PN) barrier in silicon technology. Phase noise can be scaled by resorting to the multi-core approach, provided the mismatches among coupled oscillators are carefully considered. A 16-core 20-GHz (voltage-controlled) oscillator demonstrates −130 dBc/Hz at 1 MHz offset minimum phase noise with 850 mW from 2.4 V supply. A more elegant and efficient approach is then introduced. Leveraging the series resonance of a tank, the remarkably lower impedance at resonance considerably increases the tank active power, thus enabling a remarkable improvement in spectral purity. Two 10 GHz BiCMOS VCOs exploiting the concept are presented. The measured minimum phase noise is −138 dBc/Hz at 1 MHz offset with 600 mW from 1.2 V supply. Experimental results demonstrate the lowest phase noise ever reported by fully integrated RF oscillators in silicon technology.