High strength concrete is being used increasingly in major civil engineering and building projects. Measurement of the in-situ strength is often required as a means of determining early age strength development and form work stripping times, or, for verifying structural performance. A number of techniques for the estimation of the in-situ strength of high strength concrete have been examined. The use of temperature matched cured test cubes was found to give the closest correlation with in-situ strength although ultrasonic pulse velocity measurements and pull-out tests were useful techniques. One aspect of testing high strength concrete that emerged as significant, was the need to base calibration or correlation curves on specimens sealed against moisture ingress or moisture loss. rather than on water-cured specimens. Two high strength concrete mixes incorporating different aggregate types (with mean 28 day cube strengths of 90 MPa and 100 MPa) were investigated in large-scale test items. A comparison of maturi measurements, UPV and pull-out tests is given together with discussion of the nature of in-situ strength development in high strength concrete.