Upstream oil and gas fields pose unique challenges to scientists and engineers tasked with characterizing and managing environmental risks. These challenges may include complex hydrogeologic settings, unknown or often incomplete knowledge regarding chemical releases, and complex mixtures of chemicals such as petroleum hydrocarbons, salts, and naturally occurring radioactive materials. Furthermore, upstream oil and gas fields offer unique health, safety, and logistical challenges due to their large size and remoteness, which can pose significant site access challenges. Lastly, environmental regulation of upstream oil and gas fields in the U.S. has been left largely to the states, resulting in different environmental policies in different states. However, these challenges also provide opportunities for creative problem solving and innovation.