Abstract : In the field of pharmaceutical chemistry the crystallization of the active ingredient or intermediate is an important stage, it is going to control the physico-chemical properties of the solid obtained. The first stage of the crystallization, the nucleation, can be slow, difficult or disturbed by the appearance of another unwanted phase, solid or liquid. The obtaining of the first crystal and the research for new polymorphs need long and expensive studies. Our aim is to rationalise these researches on crystallization conditions, for that we propose, in this PhD thesis, the study and the development of new crystallization techniques on small volumes (from nL to mL) under an external field. As external fields we mean under an electric field, a light or ultrasounds irradiation. We will be able to enhance nucleation. We want to control spatial and temporal localisation of the nucleation event in metastable solutions, in order to observe and better understand the mechanism of the first nucleus appearing.