The purpose of this paper is to describe the non-normal surfaces of degree 5 and 6 embedded in the complex projective space P", with n i> 4. The idea is to study the normalization of such a surface, and then to find how a non-normal surface can be obtained from its normalization.Let Yc Pn be a surface of degree d, linearly normal and not contained in any hyperplane of P~. If d ~< 4, the list of all such surfaces is known (cf. 1-23], [24]). If 5 ~< d ~< 8 and Y is smooth, the classification is also known (cf.[15], 1-16]). The list of all normal singular surfaces of degree 5 and 6 is given in [10] and [11]. The purpose of this paper is to describe the non-normal surfaces of degree 5 and 6 embedded in P~.