Although the United States has been stimulating well production with hydraulic fracturing (“fracking”)1 since the 1940s [1], high-volume hydraulic fracturing (HVHF) combined with horizontal drilling is a relatively recent [2, 3] development with potential to adversely impact human health [4], environment [5], and water resources [6], with uncertainty about impacts and gaps in the data on HVHF compared to conventional drilling techniques [7]. Part of protecting environmental and public health is identifying potential risks before licenses are issued and drilling operations proceed. To this end, two case studies, focusing on the environmental impact assessment (EIA) procedures of California and New York, are analyzed in this paper. Both states have histories of strong environmental protection law and policy [8–10] and legally require an EIA to be conducted before development of HVHF sites [11, 12], an outgrowth of the 1969 federal National Environmental Policy Act (NEPA). New York State conducted what appears to be a thorough EIA [13] and concluded that as there were too many gaps in the data on HVHF, fracking could not proceed. California’s EIA, which was less extensive, and did not consider health impacts [14], concluded that HVHF could proceed, relatively unabated. A comparison of these cases illustrates that the processes designed to ensure adequate identification, monitoring, and assessment of environmental impacts are prone to differences [15]—an outcome of the fact that laws governing HVHF in the US are not consistent across, nor controlled at, the federal level [16, 17].