The largest-ever exercise to validate dynamic thermal simulation programs (DSPs) of buildings has recently been completed, It involved 25 program/user combinations from Europe, the USA and Australia, and included both commercial and public domain programs, Predictions were produced for three single zone test rooms in the UK. These had either a single-glazed or double-glazed south-facing window, or no window at all. In one 10-day period the rooms were intermittently heated and in another 10-day period they were unheated. The predictions of heating energy demands and air temperatures were compared. The observed interprogram variability was highly likely to be due to inherent differences between the DSPs, rather than the way they were used. Predictions of the difference in performance of two rooms were no more consistent than predictions of the absolute performance of a single room, By comparing the predictions with the measurements and taking due account of experimental uncertainty, the DSPs that are likely to contain significant internal errors are distinguished from those which, in these tests, performed much better. The likely sources of internal error are discussed. It is recommended that empirical validation exercises should consist of an initial blind phase in which program users are unaware of the actual measured performance of the building, and then an open phase in which the measurements are made available. The work has produced five empirical validation benchmarks, which have significant practical benefits for program users, vendors and potential purchasers. There is considerable scope for improving the predictive ability of DSPs and so suggestions for further work are made.