Abstract: A method of determining the legitimacy of a base station (20) in a cellular telecommunications network by an electronic communication device (10) capable of connection to the cellular telecommunications network comprises the electronic communication device (10) determining an expected signal strength for transmissions from the base station (20) based at least on an expected geographical location of the base station relative to a current geographical location of the electronic communication device (10). The electronic communication device (10) measures an actual signal strength of transmissions from the base station (20) and compares the actual signal strength to the expected signal strength. The electronic communication device (10) determines that the base station (20) is illegitimate if the actual signal strength exceeds the expected signal strength by at least a predetermined amount.