There is disclosed an electrostatic discharge (ESD) device tester and a method of operating the tester. In an embodiment, the method comprises operating the tester by uniquely identifying an ESD device to be tested using identification means provided on the tester; taking at least one test measurement of the uniquely identified ESD device using testing means provided on the tester, the testing means being configurable in dependence upon data associated with the uniquely identified ESD device; and storing the at least one test measurement in a storage means provided in the tester. A running average of test measurements for the uniquely identified ESD device may be stored on the tester in order to compare a test measurement against the running average. A test is repeated if a test measurement falls outside of a predetermined range of the running average.