Minimum Ignition Energy (MIE) of dust clouds is required to assess the electrostatic ignition risk. Recent studies are reported that indicate that the test methods in use to determine MIE give markedly different values. The use of MIE to define the incendivity of discharges from conductors and non-conductors is discussed and the value of data from present day tests is considered. (C) 2001 The Institute of Physics. Published by Elsevier Science Ltd. All rights reserved.