Formation water or hydrocarbons occurring at surface and subsurface locations away from their point of origin are often referred to as “stray fluids.” Efforts to identify the sources of these fluids have provided important insights for optimizing hydrocarbon exploration and production. With the rapid growth in hydraulic fracturing operations, the source of associated fluids is becoming the focus of scientists and environmental regulators. Many geochemical techniques are available for fingerprinting stray fluids, but the information from traditional approaches can be difficult to interpret in some oil and natural gas settings. New isotopic techniques, using signatures of 18O, 2H, 13C, 87/86Sr, and others, are now placing better constraints on the interpretation of stray-fluid origins. These new isotopic fingerprinting methods are being used by the hydrocarbon industry to solve problems and safeguard public health.