In this thesis, we investigate experimentally as well as theoretically collective effects in dilute clouds of cold atoms. In order to study the competition between cooperative effects and strong localization, we then implement a dipole trap that allows us to compress the cloud to dense regimes. The first chapter shows how a system of N atoms interacting via the electromagnetic field gives rise to cooperative effects: superradiance, subradiance, collective Lamb shift. Considering the situation where atoms are driven by an external laser field, we point out how collective effects occur and compute the cooperative radiation pressure force acting on the center of mass of the cloud. Subradiance is then studied by considering the system relaxation after switching off the driving laser field. The second chapter describes the experimental investigation of cooperative effects through measurements of the collective radiation pressure force. These measurements show good agreement with the theoretical model we have previously developed. The last chapter explores the experimental realization of a blue detuned crossed dipole trap whose size can be dynamically adjusted enabling us to compress the cloud to reach high density regimes.
