The numerical simulation of systems involving Yukawa interaction y(r)=exp(-alphar)/r (e.g., colloids, dusty plasmas,...) needs some caution in the case where the potential cannot be neglected on the boundaries of the cubic simulation cell [i.e., when y(L/2)-L side of the cube - is larger than the absolute uncertainties wanted for the thermal average of the energy]. In that case the usual minimum image convention fails and it is necessary to introduce Ewald sums similar to those used for Coulomb systems. In this study we derive the expression of the Ewald sums associated to Yukawa interactions and discuss the numerical errors induced by their truncation.