This text presents the current knowledge of environmental colloids and includes reviews of the current understanding of structure, role and behaviour of environmental colloids and particles, whilst focussing directly on aquatic systems and soils. In addition, there is substantial critical assessment of the techniques employed for the sampling, size fractionation and characterisation of colloids and particles. Chemical, physical and biological processes and interactions involving colloids are described, and particular attention is paid to quantitative approaches that take account of particle heterogeneity and polydispersity.