As the size of optical devices shrinks to micro/nano-scale, hot electrons in metallic micro/nano-structures come to the front; here hot electrons refer to the excited electrons in a metal lattice. They play an important role in researches and applications involving the harvesting and conversion of light energy. Knowing the characteristics of hot electrons in metallic micro/nano-structures is of great importance for the design of corresponding optical devices. To comprehend the properties of photon-excited and plasmon-induced hot electrons in metallic micro/nano-structures, we discuss the detailed excitation and relaxation processes of these electrons. Understanding such mechanisms is the first step to the development and applications of micro/nano-scale optical devices based on hot electrons.