Micronutrients are involved in all metabolic and cellular functions. Plants differ in their need for micronutrients, and we will focus here only on those elements that are generally accepted as essential for all higher plants: boron (B), chloride (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), and zinc (Zn). In this review, we focus on the major functions of mineral micronutrients, mostly in cases where they were shown as constituents of proteins, making a selection and highlighting some functions in more detail.