Terms for indicating the relationship between organisms and their substrates are necessary for effective communication in ecology and palaeoecology. Usage of terms denoting these relationships are discussed and suggestions for more meaningful usage given. Suggested/recommended usage attempts to adhere as closely as possible to the meanings of the terms as originally proposed (e.g. symbiosis). Distinction between substrate and medium is important. Although substrate is categorized as firm and loose, detailed petrographical and/or sedimentological analyses of substrates are needed in both ecology and palaeoecology. A symmetrical classification of organism-substrate relations defining recommended terms which cover broad categories provides a terminology which hopefully is flexible and serves a useful function.