Abstract : The laser source for the gravitational wave detector VIRGO is designed to deliver a stable, continuous, single frequency 20 W beam at 1.064 μm. I first review the main requirements for the design of the laser source. Then I give the architecture and techniques used to stabilize the laser beam, and the performances of the system: until July 2002 the laser system was able to deliver a power of 2.7 W, with a frequency stability of 500 Hz rms, with a duty cycle greater than 90%. In the following part, I study the different noise sources and the way how they couple to the system in order to explain the lack of stability of the laser. I show that the local control contributes to increase the frequency noise and the power noise up to 10 Hz. Some mechanical resonances between 10 Hz to 1 kHz are identified, which can be eliminated by simplifying the corresponding mechanical structures. The resulting improvements should allow the system to fulfil the requirements.