To deal effectively in the course of a single lecture with the subject of light alloys for aircraft is, to-day, practically an impossibility. Actually, the various alloys of aluminium that are put forward for consideration by the aeronautical engineer do not vary amongst themselves to such an enormous extent as to make one or other of over-riding interest, and much the same might be said of the alloys of magnesium. Despite this position, so much of interest is occurring in both fields, and so many alloys are being put forward having some characteristic or another of real interest, that the aircraft engineer, and in fact any other engineer, could readily be excused if he felt somewhat nonplussed when he surveyed the multitude of materials that are offered for his employment. I am relieved, therefore, that your Council decided that this lecture should give special prominence to the alloys of magnesium. I am assuming that by this they mean those alloys which consist of magnesium in a great preponderating proportion, but I feel that the time is appropriate also to make some reference to those alloys which consist principally of aluminium, but which possess definitely useful properties in consequence of the presence in them of certain relatively small proportions of magnesium. I propose, however, first to consider the alloys rich in magnesium, and will endeavour to provide some data of a rather fundamental type which may be of definite service to the aeronautical engineer when he is engaged in comparing the relative importance and value of the magnesium alloys made available to him in commerce.