When a dispersion of fine particles is concentrated, the product can contain clumps which arise from the aggregation of the particles. There are several drivers: sedimentating, drying, filtration, forcing the particles together to produce agglomerated structures which are much larger than the primary particles. The problem of understanding this phenomenon is twofold: on the one hand, it is difficult to measure aggregates in a concentrated slurry; on the other, there is no theory to predict when aggregates should form in an apparently-stable dispersion. This paper describes a new experimental method for measuring aggregates in concentrated suspensions, showing that the aggregation phenomenon can be followed over a wide range of experimental conditions. In particular, the results show that the aggregates exist at small concentrations in ostensibly stable dispersions even before concentrating takes place. Colloids based on polymers, ceramics, biological cells and emulsions all showed this aggregation effect. We have called these aggregated structures "multiplets" to distinguish them from the more normal flocs produced by destabilising the colloid. A theory of aggregation is proposed to fit the experimental results. This theory is based on the idea that multiplets form as a consequence of small adhesion forces between particles immersed in liquid; a molecular dynamics simulation using this concert of adhesion forces is used to demonstrate the formation of multiplet material at low concentrations. The theory seeks to show how the size of multiplets should vary with adhesion and with particle concentration.