The estimation of the strength of concrete in an existing structure, or the assessment of compliance of concrete supplied to a project in terms of compressive strength remain the Engineer’s Holy Grail. The procedures to undertake these assessments are complex and need to be approached with caution. NDT methods are often used in conjunction with cores to minimise the damage to a structure when conducting assessments and it is crucial that these are carried out correctly and in the right places if meaningful results are to be obtained. BS 6089 also includes a method to confirm that two individual concretes can be considered to come from the same population, by assessing strength on each by NDT and considering the standard deviation of the results. In this way, an area of suspect concrete can be compared with an area of concrete already accepted, without the need to resort to core samples. This paper reviews the methods and techniques available to the engineer and highlights the problems and pitfalls in conducting such assessments.