This book is the result of a body of research in the field of microwave applications. The proposed chapters are the results of ten years of research supervised in our laboratory. From the characterization of composite materials to design techniques and measurements on microwave circuits: patch antennas, filters and the design of RFID transmission chains. Also, special interest has been given to the interactions of electromagnetic waves with biological tissues; in particular the human head to assess the thermal effects of mobile telephony on the human body. Thus, this book is a scientific contribution to the development of research axes in microwave applications.