The purpose of this dissertation is to go through the differents approaches of fantasy both in Freud s and Lacan s theories. Freud presents two basic dimensions of fantasy: at first, a representative dimension where fantasy is articulated to infantile sexuality, to desire s realization, to the pleasure principle and repression. Later on, a drive dimension, represented by the fundamental fantasy of beating, began to be priorized by the author. Freud emphasized a scar of the Oedipal castration, always present in everyone, which remains apart of neurosis structure, claiming for a specific work of construction in analysis. We present how Lacan, based on the last Freudian conception of fantasy, built the formula of the fundamental fantasy: (subject barred in object search a). This special fantasy, in the authors opinion , is a construction that allow us to cope with Other s desire. We examine different aspects that fantasy can assume in neurosis or in perversion. Finally, we introduce how the author relates fantasy to the end of analysis,emphasizing its crossing instead of the relief of the symptoms, and the changes on the position of the analyst and on the treatments conduction which are a consequence of this idea.