This paper reviews the collective experience of monitor calibration studies in adults and seeks to answer the following questions: What has been done? What have we learned? What could be done to further enhance the comparability of results from future calibration research? Calibration studies in adults have typically used oxygen consumption as a criterion measure, similar types of source activities, and linear regression to obtain prediction equations that calibrate the activity counts to measured activity intensity levels. However, the methodological diversity of these studies has produced a great deal of variation in the resulting prediction equations and cut points, even when using the same monitor. Thus, data obtained from a relatively robust activity monitoring technology that captures many dynamic physical activities reasonably well have been splintered by the calibration process into a wide range of summary measures that are much less comparable than they could otherwise be. This heterogeneity in calibration results reduces our ability to interpret data obtained from accelerometers between different research groups, across the life span, between populations, and probably between the different monitor types. This report reviews and critiques methods typically used for developing calibration equations and determining activity count cut points for identifying specific intensities of PA among adults, and it highlights the need for flexible research methods that can enhance the comparability of results from future calibration studies.