A density measurement system for measuring the density of a material within a region, the density measurement system comprises: a plurality of electrodes arranged around the region; an energization source arranged to apply an electrical signal to at least one of said electrodes; a monitor arranged to monitor an electrical parameter at at least one of said electrodes, the monitored electrical parameter being caused to change in response to flow of electrical current within the region; and a processor arranged to: generate data indicative of the complex impedance of the material within the region based upon the monitored electrical parameter; and generate data indicative of the density of the material based upon the data indicative of the complex impedance of the material.