This book mainly comprises studies of convoluted array elements derived from dipoles, cross dipoles and strips, supported by theoretical predictions and experimental results. Designs and design procedures of single and multilayer Frequency Selective Surfaces (FSS or dichroics), with applications in the millimetre wave band, are also described here. Numerical programs based on the well known modal analysis approach were mainly used to provide most of the theoretical results. An extension of the Fabry-Perot Interferometer concept is introduced to help the analysis of multilayer FSS.