It is accepted that nutrition is important in the development and function of the immune system. However, the study of nutritional immunology is a relatively new discipline. In part this may be due to difficulties in defining the effect of nutrition on immunity, as an immune response is the result of an orchestrated cascade involving many cell types. This paper reviews basic concepts in immunology and discusses some of the immunological methods available to answer nutrition questions. The review focuses on the application of methodologies designed to acquire, to describe, and to assess functionally the T-lymphocyte component of the cell-mediated immune function. Functional measurements in vivo (delayed type hypersensitivity response) and in vitro (lymphocyte blastogenesis and metabolism) and immune mediators (cytokine production) have been used to determine the effect of nutrition on lymphocytes. The challenge of the future is for nutritionists to utilize many of the new concepts and techniques in immunology to design experiments that will ultimately lead to defining diets to modulate immunocompetence in health and disease.