In this chapter, I am not setting out to provide a definitive account of the classification of particulate and dissolved matter in water bodies: such a task is impossible at present. It is recognized that there is a need for a working system of classification so that quantities of material within categories can be compared both within and between locations, but there is considerable variety in the categories employed. Some areas (e.g., what the categories within dissolved organic matter should be) are especially problematical; others (e.g., the use of binomial classification of plants and animals) create much less debate, at least in general usage by nontaxonomists.