When the size of an object reaches a critical length with respect to a particular physical parameter, "size effects" lead to anomalous behaviour compared with bulk material. Electron diffraction and high-resolution transmission electron microscopy (HRTEM) were used to study some of them. For instance, the melting temperature of nm-sized gold crystals is lowered by several hundreds of degrees. Small particles of face-centred cubic metals adopt a structure based on multiply-twinned tetrahedral subunits or icosahedral-like stacking and their lattice contracts. Gold nanocrystals are also observed to sinter at room temperature within fractions of seconds and to change their structure and orientation. Does this behaviour result of a new state of matter, named quasi-melting, from thermal fluctuations or from energy income brought by the electron beam? (C) 2003 Elsevier Science B.V. All rights reserved.