The paper presents selected aspects of engineering experience in using Non-Destructive Testing methods for “in-situ” technical evaluation of concrete structures. Attention was paid on the fact, that proper performance of NDT allows not only evaluating the basic concrete properties, but makes also possible to answer several questions, which seem to be crucial for engineering practice. Among other things, the following problems have been discussed: reliability of CAPO-Test technique for estimating “in-place” compressive strength and conditions for proper evaluation of carbonation process in existing concrete structures, as a crucial parameter for their durability assessment. The theoretical considerations were supported by a number of examples from engineering practice.