Building a number of systems for work that use low power microcontrollers and radios that spend much of their time switched off therefore it's difficult to build up a picture of current consumption over time and therefore by measuring the voltage dropped over a small value precision resistor using a digital storage scope, we can have a look at the area under the trace to extrapolate the cumulative current draw. This is a low side current shunt and therefore needs to be connected between the power supply ground and the ground connection of the circuit under test. It therefore works best on battery powered equipment where it doesn't matter that ground is slightly higher electrically than the battery ground; but be aware when measuring. A good overview is here: http://www.eetimes.com/document.asp?doc_id=1279404