Rapid growth in unconventional oil and gas (UOG) has produced jobs, revenue, and energy, but also concerns over spills and environmental risks. We assessed spill data related to 31,481 UOG wells in Colorado, New Mexico, North Dakota, and Pennsylvania between 2005 and 2014. We found between 2 to 14 percent of wells reported a spill each year. Median spill volumes ranged from 0.5 m3 in Pennsylvania to 4.9 m3 in New Mexico; large spills exceeding 100 m3 have also been reported. Spill rates differed with state reporting requirements. Seventy-five to 94 percent of spills occurred within the first three years of well life when wells were drilled, completed, and had their largest production volumes. Across all four states, 50% of spills were related to storage and moving fluids via flowlines. Enhanced and standardized regulatory requirements for reporting spills could improve the accuracy and speed by which these analyses could be undertaken to inform future policy and regulatory efforts, including the identification, prevention and mitigation of spill risks and potential environmental damage. As UOG expands within and outside of the U.S. our results and interactive spills data visualization tool can direct efforts to reduce the likelihood of spills.