The law of weaponry does not simply prohibit or restrict the use of certain weapons, means and methods of warfare but goes on to set out rules designed to ensure that States comply. Those rules on compliance include a kind of self-policing rule in which States are required themselves to assess the legitimacy of what they plan to acquire or produce. It is the purpose of this contribution to consider a number of questions. What exactly are States obliged to assess; what are the criteria against which the assessment should be undertaken, how should the assessment criteria be applied to cyber weapons, autonomous attack capabilities, space weapons and nanotechnology and what does all of this tell us about the relevance and adaptability of modern weapons law to the technological demands of the 21st century? The author does not presume in what follows to seek to predetermine how States should interpret the rules by reference to these technologies. Rather, he seeks to set out his own understanding of the technologies concerned and of the relevant law and to explain his own views on how they interrelate.