Use of microwave for investigating the integrity of structural elements has been established as a nondestructive evaluation (NDE) method in civil engineering, especially for detection of invisible damage such as voids and cracks inside concrete and debonding between rebars and concrete caused by corrosions and earthquakes, and objects such as steel rebars or dowels. The authors have found that focused microwaves are much more effective than unfocused ones and thus developed two systems for focusing microwaves: one using dielectric lenses and the other using antenna arrays. The former is referred to as a passive system as the focusing point has to be manually adjusted by moving the lenses, while the latter is an active system as focusing is automatically performed by software without moving the antenna arrays. This paper presents the numerical and experimental results, including measurement of dielectric properties of related materials, the passive microwave imaging technology using dielectric lenses, and the active microwave imaging technology using the antenna arrays with 3-dimensional image reconstruction algorithms. In addition, this paper presents two techniques, multi-frequency technique and incident field extraction technique, for further enhancing the performance of the microwave imaging technology in the field.