This book focused on effectiveness of NDTs for detection of concrete deterioration or defects. Tests were performed to evaluate the feasibility of using Impact-Echo Method and Ultrasonic Pulse Velocity Test in detecting defect and determining its depth during the early age concrete (between days 3 to day 28). Tests were performed on two RC Slabs Grade 30 and 40 at day 3,7,14 and 28 with a fabricated void at a known location. Tests were also performed to examine theoretically the relationship between the porosity of concrete at each stage and different strength with the accuracy of void depth detected. Both methods could detect defect in specimens during the early age. However, in the determination of depth of defect, Impact-Echo Test gave more accurate results. It is concluded that both methods can be used to assess the in-situ properties of concrete or as means for quality control on site since both tests could detect defects from day 3 and the accuracy ranges 55.75% to 99.05% from day 3 to day 28.