The genetic and nongenetic factors affecting the elemental composition of animal and human tissues are presented. Genetic factors include: (1) some well-known inherited diseases that affect copper, zinc, iron, magnesium, manganese and selenium metabolism; (2) animal species differences and (3) breed differences within animal species. The nongenetic factors include regional variation, dietary concentration of elements, interactions between elements, dietary concentration of other nutrients, chemical form of elements, age, sex, hormones, pregnancy, disease, temperature and altitude.