Abstract: The fundamentals, general models and mathematical formula of environmental isotopes trace ~(13)C, ~2H and ~(15)N in the agricultural sciences are reviewed. Carbon isotope discrimination is used to indicate photosynthetic pathways in plants, assess water using efficiency of plants, investigate turnover of soil organic matter and evaluate the assimilation of different feeds. Hydrogen isotope discrimination is used to trace water resources for plants and nitrogen isotope discrimination is used to determine nitrogen fixation and study N cycling.