The quality assurance during and after construction of new structures and after refurbishments, the characterization of material properties and damage due to time and environmental influences are more and more becoming a serious concern. In recent years, innovative Non Destructive Testing (NDT) techniques, applicable for the assessment of existing civil structures, have become available for in situ analysis on concrete and masonry structures, but they are still not established for regular inspections. In addition, in the civil engineering field, repairing and anti-corrosion protection of the building structure is one of the most important applications of polymer composites. Nonetheless, the adhesion between overlays and concrete or masonry substrate is a concerning factor affecting the reliability and durability of repair. During the change of environmental conditions (temperature and humidity) after curing, the need exists for adhesion monitoring in the case of large area objects like industrial floors, bridge decks, injected concrete structures. In this chapter two main objectives will be discerned. The first relates to the application of sonic and ultrasonic testing techniques to measure in situ the development in time of the mechanical properties of concrete and polymer composite. The second deals with the assessment of damage and defects in concrete or masonry, and between concrete or masonry and applied polymer composite by means of sonic and ultrasonic techniques. Different data acquisition procedures and methodologies together with example applications will be described including tomography, scanning acoustic microscope, transmission and surface measurements. Further, an impact-echo example application on post-tensioned concrete will be described. The importance of data visualisation in form of 2-D images will be discussed. All these are monitoring techniques that can be applied in situ after their validation in laborator
