Abstract The study of trace fossils is a useful and necessary part of any core study and should be joined with other paleontologic, petrologic and sedimentologic data to give the most plausible geological interpretation. For the petroleum industry, cores, in many instances, are the best or only source of data for determining the paleogeography and paleoenvironment of subsurface deposits. Consequently, the study of trace fossils in cores becomes especially important in order to acquire the maximum information from costly samples. Such studies can be applied either to single facies or to a more general basin analysis. In the course of an ichnological study, sedimentologic, stratigraphic, structural, paleontologic, paleoecologic, paleogeographic and paleoenviron-mental data may be included. Sedimentologic events that are influenced by biologic activity include: 1) alteration of grains by ingestion; 2) disruption of fabric and creation of new fabric, which may reduce or increase permeability and porosity within beds or transmissibility between beds; 3) production of sediments by organisms, and 4) trapping of sediment by organisms (e.g., stromatolites, sabellarid worms; see Chapter 2 ). Some sedimentologic interpretations that can be made by investigation of certain traces include: 1) initial history of lithification; 2) rates of deposition; 3) current energy levels (Howard, 1964; Spencer, 1976); 4) relative amounts of erosion and deposition; 5) coherency (stability) of the medium burrowed; and 6) relative degrees of compaction (see Chapters 2 and 6 ). Where the rocks are otherwise “unfossiliferous”, trace fossils may provide biostratigraphic information (Seilacher, 1970)