The first concrete pressure vessel for a gas-cooled nuclear reactor in Great Britain was constructed at Oldbury in 1962 and commissioned in 1967. During its construction, 350 vibrating wire strain gauges were cast into the concrete and data have been collected from these gauges over the twelve years of the reactor's operational life. This paper gives an assessment of the measured strain histories, in which comparisons are made with elastic and creep analyses. From the results presented it can be seen that the total strain is dominated by the non-elastic strain. In one example where the elastic strain under operational loading is about 100 × 10 −6 strain units, the total strain after twelve years is about 400 × 10 −6 . Creep strain forms the major part of the 300 × 10 −6 difference and its derivation is of great importance. It is a function of temperature which will vary with operational requirements but will also have local variations. However, the maximum stress on most of the vessel occurs under pre-stress only. The vessel is under this loading for about two years before start-up and it can be seen from the strain histories that the majority of the creep has occurred during this period. If the subsequent temperatures are modest then later creep and any strain due to stress redistribution (caused by differential creep) will also be modest. This has proved to be the case. A nuclear reactor is subjected to scheduled shut-downs at about two-year intervals. Opportunities are therefore available to measure the (almost) elastic changes that occur on depressurisation and subsequent restart and due regard can be taken of the changes in Young's Modulus with age. The regular monitoring of the strain gauges and other transducers in the reactor vessels provides evidence that the vessels are behaving within their original design margins. The embedded instrumentation has also been an essential research tool to assess the requirements for material property data and methods of calculation.