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An enduring piece of legal wisdom contends that the value of court opinions depreciates as they age and a variety of factors lead some cases to depreciate faster than others. We measure depreciation as the change in the frequency with which Supreme Court cases are cited as a function of their age. We then examine whether the rate of depreciation varies systematically based on ideological considerations, opinion characteristics, and citation history. Our results indicate, first, that precedents depreciate rather quickly and, for example, depreciate about 81 percent and 85 percent between their first and 20th years of age at the Supreme Court and courts of appeals, respectively. Second, few of the variables in our analysis have any appreciable influence on the pace of depreciation. Two variables capturing the citation history of a case have the most notable influence on depreciation, but even their effects are reasonably modest and somewhat short-lived. Third, while our study focuses on depreciation (i.e., the change in the frequency of citation over time), it also produces an important implication for citation itself (i.e., the number of times a case is cited in a given year). We show that prior studies significantly overestimate the effect of almost every variable used to explain citation rates because those variables become substantially less influential as cases age. Future studies must therefore take into account that the effect of an independent variable on citations is conditional on the age of a precedent. This study therefore contributes to our understanding of the process by which law, as observed through citations to cases, changes.