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K3 surfaces and Enriques Surfaces are two closely related objects in Algebraic Geometry. It is known that the unramified double cover of an Enriques Surface X defined by the torsion class K_X is an algebraic K3 surface. Also, conversely, if a K3 surface X admits a fixed point free involution ι, then the quotient surface X/ι is an Enriques Surface. In this talk we will examine, following Keum's work, under which circumstances a K3 surface will admit a fixed point free involution, hence, will cover an Enriques Surface and we will give some applications for this characterization.