Future antenna architectures especially for space applications are becoming more and more complex due to the need of Electronic Reconfigurability. This reconfigurability is needed in terms of Radiation Pattern, Bandwidth, Reliability, and Power Consumption. In this context, Electronically Tunable Reflectarrays and Frequency Selective Surfaces (FSSs) are particularly the hottest domains of RF design. The accurate analysis of electromagnetic(EM) scattering from such reconfigurable antenna structures is of great practical interest these days. However due to their large electrical size and complex cellular patterns specially when tuning elements such as RF-MEMS switches are also integrated within the array elements, conventional full-wave EM Modeling of such multiscale structures either fail or require enormous amount of computational resources to resolve prohibitively large number of unknowns. A novel and efficient modular numerical technique called Scale Changing Technique (SCT) addresses all these problems. The focus of this book is on EM modeling of passive and active (electronically steerable by RF-MEMS Switches )microstrip reflectarray using this Technique.