The research on advanced materials is moving at a pace to achieve better performance with limited resources and time. Electromagnetic materials made up of materials such as metals and dielectrics, when scaled down to subwavelength dimensions and when arranged in a periodic manner, begin to exhibit some unique properties that cannot be obtained or that cannot be observed using the conventional materials. These materials are known as meta-materials and have attracted the attention of the electromagnetic community because of their myriad applications. They are usually arranged in repeating patterns, at scales that are smaller than the wavelengths of the phenomena they influence. Their precise shape, geometry, dimensions, orientation and arrangement gives them the ability to influence the electromagnetic waves and achieve benefits that go beyond what is possible with conventional materials. Over the last few years, metamaterials have discovered an unprecedented rise in practical applications in various fields of science and engineering, particularly, in microwave and terahertz frequency regimes. This book will provide a glimpse into the design of filters, sensors and absorbers in microwave and terahertz frequencies. The main objective of this book is to create awareness among young researchers about this challenging and growing field of this metamaterial, and to provide them detailed theory of designing such structures.