Photosynthesis is one of the fundamental biochemical and physiological processes that distinguish plants from animals. Plants can be categorized as having one of three different photosynthetic pathways: C3, C4, or CAM. This chapter introduces the theories that provide food scientists with sufficient information to predict variations in the isotopes of these elements in plant-based foods. While understanding of natural isotope ratio variations in nitrogen, sulfur, and strontium is not yet as fully developed, it is possible to generate preliminary interpretations of variations in these isotope ratios of plant and plant-based foods and link these variations to growth condition and geography. The applications of stable isotope analysis to plant biology and food science provide a foundation for interpreting environmental, cultivation, and genetic-based variations in our plant-based foods. Understanding plant nitrogen stable isotope values can be complicated, because plant nitrogen can be taken up by roots, through mycorrhizal fungal associations, or symbiotically through bacteria in root nodules.