In the prior-appropriation water rights regimes that prevail in the arid western United States, claims to annually variable surface water flows are fulfilled on the basis of the order of their establishment. The two-step process used to establish an appropriative water right in all 17 conterminous western states creates a temporary phase, or conditional water right, that has a priority date but no actual water use. This article reviews the legal basis for these conditional water rights and demonstrates the potential uncertainty they introduce to current water users. It then presents a complete census of conditional water rights (amounts, ages, and uses) in Colorado. At the end of 2012, conditional water rights in Colorado (some over 90 years old) were equal to 61% of the perfected water rights. Many of the controversial conditional water rights in Colorado have been associated with unconventional oil production in the northwestern portion of the state; however, conditional water rights are ubiquitous across the state and across many use types. In several basins, their existence can introduce uncertainty to some of the most senior water rights holders. Nevertheless, in most of the state, the effects of conditional water rights are restricted to a relatively junior class of water users. This work quantifies for the first time the result, in one state, of a peculiar aspect of water law common across all western prior-appropriation states.