We classify Enriques involutions on a K3 surface, up to conjugation in the automorphism group, in terms of lattice theory. We enumerate such involutions on singular K3 surfaces with transcendental lattice of discriminant smaller than or equal to 36. For 11 of these K3 surfaces, we apply Borcherds method to compute the automorphism group of the Enriques surfaces covered by them. In particular, we investigate the structure of the two most algebraic Enriques surfaces.ICHIRO SHIMADA AND DAVIDE CESARE VENIANI but X 7 does. Hence, following Vinberg, we call the Enriques surfaces covered by X 7 the most algebraic Enriques surfaces.Theorem 1.0.1. The singular K3 surface X 7 of discriminant 7 has exactly two Enriques involutionsε I andε II up to conjugation in Aut(X 7 ). Let Y I and Y II be the quotient Enriques surfaces corresponding toε I andε II , respectively. Then Aut(Y I ) is finite of order 8, and Aut(Y II ) is finite of order 24.Nikulin [26] and Kondo [19] classified all complex Enriques surfaces whose automorphism group is finite. It turns out that these Enriques surfaces are divided into 7 classes I, II, . . . , VII, which we call Nikulin-Kondo type. See Kondo [19] for the properties of these Enriques surfaces. Corollary 1.0.2. The most algebraic Enriques surfaces have finite automorphism groups and their Nikulin-Kondo types are I and II.In Section 6 of this paper, we give explicit models of the most algebraic Enriques surfaces Y I and Y II as Enriques sextic surfaces.Remark 1.0.3. The Néron-Severi lattice and the automorphism group of X 7 were determined by Ujikawa [39]. Elliptic fibrations on X 7 were studied by Harrache-Lecacheux [12] and Lecacheux [21].Remark 1.0.4. Mukai [23] also realized that X 7 has Enriques involutions that produce Enriques surfaces of Nikulin-Kondo type I and II.Ohashi [27] gave a lattice theoretic method to enumerate Enriques involutions on certain K3 surfaces. He then classified in [28] all Enriques involutions on the Kummer surface Km(Jac(C)) associated with the jacobian variety of a generic curve C of genus 2. We refine and generalize Ohashi's method. Our main result, namely Theorem 3.1.9, applies to any K3 surface, and we use it in the case of singular K3 surfaces to compile Table 3.1.For some K3 surfaces X, the group Aut(X) can be calculated by Borcherds method ([4], [5]); for instance, Kondo [20] implemented it in order to compute Aut(Km(Jac(C))). We apply Borcherds method in order to calculate the automorphism group of some of singular K3 surfaces X, and to write the action of Aut(X) on the nef chamber of X explicitly. Building on this data, we enumerate all Enriques involutions up to conjugation, and, using also a result of the preprint [7] (see Section 2.9), we calculate the automorphism group of the Enriques surfaces covered by these K3 surfaces.Note that the enumeration of Enriques involutions by Ohashi's method and by Borcherds method are carried out independently. The results are, of course, consistent. We hope that these methods will be applied to many other K3 surface...