A History of the development at Cambridge of a notch tensile test for the investigation of brittleness in structural mild-steel is described. The test procedure and the methods of assessing the temperature limit at which the material becomes brittle are outlined in Part I. Investigations of material from an oil storage tank and from ships which fractured in service show that this notch tensile test gives good correlation with these casualties whatever thc nominal stress in the plates at the time of the fracture. The results of Charpy V-notch and Izod tests are given for comparison@ and it is shown that the energy absorption criterion used in these tests may be misleading (Part 11). In Part I11 experiments are described in which the tensile test has been used to predict performance in test-pieces of more complicated geometric form. Criticisms of the notch tensile test are dealt with in Part IV. In the conclusions@ attention is drawn to the fact that the designer of orthodox fabricated structures must be provided with material which is ductile at all operating temperatures and at a11 stress levels.