A systematic nonlinear analysis of differential Voltage Controlled Oscillators (VCOs) is presented. A nonlinear perturbation model of these oscillators is developed using standard models for the devices, either bipolar or MOS, which include all regions of operation. Analytical expressions are then derived allowing the steady-state amplitude, the harmonic content, and the frequency deviation of the oscillator output waveform to be predicted either in steady-state or during the transient. The stability of oscillation is also established, thus thoroughly analyzing the dynamics of RF VCOs. An application of this analysis to a MOS VCO is also provided.