A major aim of paleoanthropology is to learn what ancient behaviors were related to the acquisition, processing, and consumption of meat and when these behaviors arose. For this reason, studies focusing on purported early hominid hunting and butchery sites are important if rigorous criteria for recognizing such sites are used. Different criteria currently used as evidence of hominid involvement with ancient bones are reviewed and it is concluded that the presence of cutmarks, verified by scanning electron microscope (SEM) inspection, is the most reliable. Successful application of this criterion depends upon a thorough knowledge of the normal variations in microscopic morphology of different types of marks that are found on bones. Therefore, variations in microscopic and gross morphology within and among a large sample of known stone tool cutmarks, carnivore tooth scratches, and rodent gnawing marks are documented. The effects of sedimentary abrasion, as caused by fluvial transport of bones, are also presented. Guidelines are presented for using microscopic criteria to identify unknown marks on fossils and possible applications of this approach are discussed. Further, it is suggested that evidence of hominid carcass-processing activities can be placed into one of three ranked categories of certainty according to the type of data used. Explicitly stating the category and type of evidence used to deduce hominid activities, and by extension to define site types (i.e., butchery, kill, base camp), may improve the clarity of hypotheses about and interpretations of early hominid behaviors.