Isotope labeling is a technique used in scientific research to track or study the movement of specific molecules within a system. By substituting a stable isotope of an element (such as carbon-13 or nitrogen-15) in place of the more common isotope, researchers can trace the metabolic pathways and interactions of these molecules in biological, chemical, or environmental processes. Isotope labeling is commonly used in disciplines such as biochemistry, genetics, environmental science, and pharmacology to elucidate complex biological mechanisms and understand the behavior of molecules in living organisms or ecosystems.