The microscopic treatment of the spurious motion of the center of mass in nuclear many-body systems is reviewed. In this talk we present and compare the results of using the counterterms which are needed to fulfill the translational and Galilean invariances, and those obtained by using collective variables. The formalism is applied to the calculation of matrix elements of electroweak operators, as those which are active in the (μ⁻, e⁻) conversion process.Facultad de Ciencias Exactas