The operation and the theory of the effective-index method for the analysis of a wide range of optical waveguides are reviewed. The asymptotic results that characterize the performance of the method for rectangular waveguides and waveguide arrays are discussed. An accurate version of the method, namely the effective-index method with a built-in perturbation correction, is described. The applications of the effective-index method to nonrectangular waveguides and nonlinear waveguides are also discussed.