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The oxygen isotope composition of speleothems is a widely used proxy for past climate change. Robust use of this proxy depends on understanding the relationship between precipitation and cave drip water delta O-18. Here, we present the first global analysis, based on data from 163 drip sites, from 39 caves on five continents, showing that drip water delta O-18 is most similar to the amount-weighted precipitation delta O-18 where mean annual temperature (MAT) is < 10 degrees C. By contrast, for seasonal climates with MAT > 10 degrees C and < 16 degrees C, drip water delta O-18 records the recharge-weighted delta O-18. This implies that the delta O-18 of speleothems (formed in near isotopic equilibrium) are most likely to directly reflect meteoric precipitation in cool climates only. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation delta O-18 of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water.