Caking of powders and bulk granules and adhesion of powders on equipment surfaces is a serious problem in the food, chemical and pharmaceutical industries. While caking of crystalline particles is tackled only briefly, this publication deals mainly with caking of water-soluble amorphous particles. Caking of amorphous particles starts with stickiness of the powder surface and can result in a collapse of the powder structure. The powder bulk turns from a free flowable powder bulk composed of single particles into a massive block. The material aspects (hygrocapacity, hygrosensitivity and viscosity) of water-soluble amorphous materials and sintering are discussed. The increase of the sinter bridges during caking could be modelled through the known sinter equations and measured by carrying out experiments in a ring shear cell. The calculated sinter bridge diameter could be correlated with the strength of the caked powder bulk measured in a ring shear tester. (c) 2010 Elsevier B.V. All rights reserved.