THE TOPICS OF STRENGTH AND RELATED TESTING were covered in the previous editions of the ASTM STP 169 series by C. E. Kesler and C. P. Seiss, “Static and Fatigue Strength” [1]; by C. E. Kesler, “Strength” [2]; by K. N. Derucher, “Strength” [3]; and by P. M. Carrasquillo, “Concrete Strength Testing” [4]. The current chapter augments the topics addressed by these previous authors by introducing new developments since STP 169C was published. This chapter provides information on the nature of concrete strength, preparation of test specimens, test procedures, significant factors affecting test results, strength relationships, nondestructive and in-place strength testing including maturity and temperature-matched curing, strength under combined stresses, and fatigue strength.