In many applications it is advantageous to handle or process particles as suspensions in a fluid rather than as dry powder. In such cases one would like to manipulate and characterize their flow behaviour in order to ensure optimal performance. Several difficulties arise in the theoretical or experimental study of the systems under consideration, i.e. for concentrated suspensions. Here, the basic concepts and the major results of suspension rheology are critically reviewed, Both the continuum mechanics and the microstructural approaches an considered. The different parameters that affect the flow behaviour are systematically reviewed. Large particles as well as fine, colloidal particles are discussed, Whenever possible data reduction schemes and scaling laws are indicated, from which possible strategies for predicting flow behaviour are derived. Measurement difficulties are also included, fundamental limitations and requirements are pointed out.