It has long been feasible for terrorists to acquire the means of using nuclear, biological and chemical weapons, but they are complex and hard to control and the threat to fire them may lack credibility. Amongst conventional weapons, handheld missiles have brought about the greatest changes in terrorist tactics, but they are expensive so they need rich sponsors. Small arms have changed very little: the 1884 Maxim gun fired 13 rounds per second; so do the Armalite and the AK74. Submachine-guns have become smaller and lighter and about 150 models, some old but still serviceable, are available worldwide at very low cost. There is, however, no limit to ingenuity in improvising bombs, grenades and mortars, fired by readily available commercial radio-control and timing devices. These are likely to continue to cause most of the surprises as they are difficult to predict.