Active Galactic Nuclei (AGN) play an important role in many aspects of the modern cosmology and of particular interest is the issue of the interplay between AGN and galaxies and their strong related evolution. Many studies have been done and many results achieved in the last decade but many things remain still unclear and need to be studied using larger sets of data. The Cosmic Evolution Survey (COSMOS) is currently the largest HST survey ever undertaken (~2 sq.deg). COSMOS observations include the full coverage of the field with multi-band photometry (from UV, Optical, NIR to MIR and FIR), in combination with a multi-wavelength data-set from radio to X-rays and a spectroscopic coverage obtained using VIMOS, a multi-object spectrograph mounted at VLT. Using this powerful data-set we explored the properties of a sample of AGN and we studied their evolution also in relation with the evolution of their host galaxy. I will present some final and some preliminary results achieved in this project.