Despite having been the subject of extensive study in last 20 years for the solid state IC community, the phase noise in voltage controlled oscillators (VCOs) is still today an important research subject. The main reason is that phase noise is one of the main issues encountered during the design of a transceiver whose understanding is an essential know how for an RF designer. A second reason is that the intrinsic time variant nature of VCOs makes these circuits difficult to analyze, therefore new topologies are often proposed, claiming advantages in term of phase noise and/or dissipation that in several cases are hard both to understand and verify without a direct implementation. This lecture will start from the basics of LC VCOs and of phase noise. The phase noise will be calculated in basic topologies and the fundamental trade off with power dissipation and tuning range will be highlighted. The lecture then will continue by presenting advance VCO topologies, showing how these circuits typically aim to enhance either the current or the voltage efficiency, in order to improve the phase noise vs. power dissipation trade off.