An all-order summation of core polarization diagrams using the low-momentum nucleon-nucleon interaction V low−k is presented. The summation is based on the Kirson-Babu-Brown (KBB) induced interaction approach in which the vertex functions are obtained self consistently by solving a set of non-linear coupled equations. It is found that the solution of these equations is simplified by using V low−k , which is energy independent, and by employing Green functions in the particle-particle and particle-hole channels. We have applied this approach to the sd-shell effective interactions and find that the results calculated to all orders using the KBB summation technique are remarkably similar to those of second-order perturbation theory, average differences being less than 10%. . In microscopic calculations of effective interactions, core polarization has played an essential role, as illustrated by the familiar situation in 18 O. There the spectrum calculated with the bare G-matrix was too compressed compared with experiment, while the inclusion of core polarization had the desirable effect of both lowering the 0 + ground state and raising the 4 + state, leading to a much improved agreement with experiment [1, 2]. As pointed out by Zuker [4], the Kuo-Brown matrix elements, although developed quite some time ago, continue to be a highly useful shell model effective interaction. It should be noted that the core polarization (CP) diagrams associated with the above interactions were all calculated to second order (in the G-matrix) in perturbation theory. But what are the effects of core polarization beyond second order, and how can they be calculated? In this Letter we would like to address these questions and present an all-order summation of CP diagrams for the sd-shell interactions.