Mark, trace and structure have been in consistently used in ichnology for many years; we wish to clarify the origins and to prescribe co rect usage of these terms. The origins of the words are ancient and complex; in thetwentieth century they were given clear definitions as ichnologic terms. Seilacher (1953) defined a mark (German Marke) as a physical (abiogenic) sedimentary structure, as in the common terms sole mark, flute mark, but not bite mark or scratch mark. Trace has been defined many times; we recommend the consensus defini ion of Bertling et al. (2006) as “a morphologically recurrent structure resulting from the life activity of an individual organism (or homotypic organisms) modifying the substrate”; this includes dwel ing trace, feeding trace, bite trace. Structure, as implied in another consensus paper (Frey, 1973), is a neutral term for geologic patterns resulting from ei ther biogenic or abiogenic processes. Use of the three terms in a clear consistent manner will aid communication both among ichnologists and be tween ichnologists and their col leagues in other fields.