Since the seminal work of Friedman and Smith, obsidian hydration dating (OHD) has undergone several important stages of development and refinement. Summarizes the history of research and developments and suggests some areas for future research. Archaeologists have taken a pragmatic and empirical approach towards OHD and hydration rate determination. They would like OHD to become an absolute chronometric technique. Geochemists and material scientists have sought to understand the hydration process from an environmental, chemical, and thermodynamic perspective. As suggested by the literature, obsidian hydration appears to be a diffusion reaction and dissolution process dependent on the chemical and thermal history of the hydration environment. There are discrepancies between archaeologically derived data and theoretical models. California offers a rich database for synthesis and testing of hydration models. There are over 25 natural obsidian sources. Obsidians have been distributed aboriginally as artifacts into diverse environments. Intensive excavations have contributed to development of a number of source-specific hydration rates for single sources. Reanalysis of the California data warrants future examination to advance our understanding of obsidian hydration dating on a worldwide basis.