We propose a new parametric macromodeling technique for complex electromagnetic (EM) systems described by scattering parameters, which are parameterized by multiple design variables such as layout or substrate feature. The proposed technique is based on an efficient and reliable combination of rational identification, a procedure to find scaling and frequency shifting system coefficients, and positive interpolation schemes. Parametric macromodels can be used for efficient and accurate design space exploration and optimization. A design optimization example for a complex EM system is used to validate the proposed parametric macromodeling technique in a practical design process flow. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2012.