Abstract : This tesis is divided into three parts. The first part is a study of the linear stability problem and of the effect of turbulence on the dynamo onset. The turbulence acts as a multiplicative noise on the magnetic field and other similar instabilities are studied both analytically and experimentally. The second part is a theoretical study of the nonlinear problem, close to the onset of instbaility. Using a particular example, we derive an amplitude equation that describes the saturation mechanism of the magnetic field. The result is generalized to other flows and various scaling laws for the magnetic energy at saturation are given. The last part is an experimental study of a swirling flow of liquid sodium made by the Vob Karman Sodium experiment (VKS). Measurements of the induced magnetic field are presented and show the amplification mecanisms for this flow.