Dating archaeological sites, cultures or phases by the obsidian method is still experimental, but shows promise provided the archaeologists understand the present limitations. Two categories of factors cause possible dating errors: the technical or geological aspect discussed by Friedman and Smith, and the archaeological context. For the establishment of valid rates of hydration and for the archaeological evaluation of the method, preferred samples are those datable by independent means, such as radiocarbon, dendrochronology, historical records, rates of refuse accumulation, contemporary calendars, or correlated positively with complexes or seriated sequences that are so dated. A few samples used in the preliminary part of the study do not meet these rigid standards and make the method look poorer than it is, but a table contains the raw data on all specimens tested to date. The principal source of potential error at our present state of knowledge is the frequency with which artifacts of older manufacture were re-used by a younger (more recent) culture. Evaluations of the data presented in the table and most of the results are discussed by geographical area. For the tropical region where the annual temperature range is minimal and where large obsidian samples from village refuse are available, the hydration rate appears to be a fairly accurate method of dating. Lack of conformity of obsidian dates with archaeological dates for the Maya region and the Southwest, however, emphasizes the need for further research using larger series of artifacts and for a continued examination of technical factors. Objects of modern manufacture can be identified with little difficulty, offering a simple means of detecting fakes or copies.