The measurement of depth is a central concept for IODP activities. Use of the units meters below sea floor (mbsf) and meters below rig floor (mbrf) in ODP/IODP is inadequate because innovations in the methods by which depth is measured or calculated have progressed. Whereas the traditional units are still allowed in IODP publications, sufficient documentation is needed to explain the differences in meaning and relationship between depth scales. The reason for documenting the IODP depth scales is to provide scientists data and sample materials with documented units and methods for capturing depth information, so that hey will be able to perform shipboard duties, write reports and understand IODP literature. Use of these depth scales should also support the use of IODP data in depth mappings, analytical software tools, core visualization software and applications. The issue of depth in IODP (or any other drilling program) is complex. Depth is measured and used in many different ways. Increased documentation of the differences in the relative scales by which we report depth allows crosscorrelation and integration (e.g., core, log and seismic correlation) between different data sets that allow greater understanding of the subseafloor. This document addresses the relationships between the different IODP depth scales and their relationship to the legacy system. This document presents the sources and magnitudes of errors associated with the different depth scales, the methods by which distances are measured and/or derived, the datum to which the depth is relative, the units in which the distance is measured, and the relationship between the different depth scales.