A review of the obsidian hydration rates proposed for California obsidian sources raises questions of both practical and theoretical nature about the use of obsidian dating. Obsidian hydration rates so far proposed require careful evaluation and checking against empirical data: routine use of the preliminary rates ignores cautions raised by the investigators and can lead to inaccurate chronological determinations. Used with discrimination, however, obsidian dating does yield chronological ages in which there can be a reasonable degree of confidence.