In this thesis, coupled inductors have been applied to multiphase DC-DC converters. Detailed analysis has been done to investigate the benefits of directly coupled inductors and inversely coupled inductors, compared to conventional uncoupled inductors. In general, coupled inductors for multiphase DC-DC converters have inherent benefits such as excellent current sharing characteristics, immunity to component tolerance and reduction in current control complexity. Specifically, by employing directly coupled inductors for multiphase DC-DC converters, overall current ripple can be effectively reduced, compared to that of uncoupled inductors. For inversely coupled inductors, phase current ripple can be reduced if operating points and coupling coefficients are carefully chosen. As for small-signal characteristics, inversely coupled inductors have the advantages of broadening the bandwidth of multiphase DC-DC converters and being more immune to load variation at low frequencies. On the other hand, directly coupled inductors have the benefit of low sensitivity to input variation at high frequencies. In addition, the proposed new structure for multiphase DC-DC converters has excellent current sharing performance and reduced current ripple. Computer simulations have been done and hardware prototypes have been built to validate the concepts.