This chapter develops the readers’ skills of testing, inspecting, detecting the defects, and preventing their occurrence to assure the quality of castings. The chapter begins with the importance of mechanical testing of castings for quality assurance (QA). Here both destructive testing and non-destructive testing (NDT) are defined. Tensile testing (including stress-strain curves), hardness testing (Brinell hardness test, Rockwell test, Vickers test, and Knoop test), impact testing, and fatigue testing have been thoroughly explained with the aid of labeled sketches along with rigorous engineering analysis. The applications of non-destructive testing (NDT) to detect casting defects have been presented with particular references to the magnetic particle test, dye penetrant test, ultrasonic test, and the radiographic NDT examination, including their mathematical modeling. The casting defects (misrun, cold shut, hot tears, shrinkage cavity, blow holes, pin holes, metal penetrant, mismatch, and porosity) have been explained, and their remedies have been recommended for quality assurance (QA) in both sand castings and die castings. This chapter contains 40 worked numerical examples, 18 diagrams, 27 formula/equations, 4 data tables, and 20 MCQs with their answers given at the end of the book.