I have recently been involved in discussions with three countries that are considering *huge* new investments to introduce lots of new technologies in their primary and secondary education systems. Such discussions typically focus quite a bit on what technologies will be purchased; what additional products, services and support will need to be provided if the technology is to be used effectively; and how to pay for everything. Increasingly (and encouragingly), there is also talk of how to measure the impact of these sorts of investments. To measure 'impact' (however you choose to define it), you of course need to know what has actually happened (or not happened). When you are putting computers in all schools, or rolling out lots of new digital learning content, or training lots of teachers, how do you know that these sorts of things are actually taking place?