Nuclear energy has had two most significant applications, nuclear power and nuclear weapons. In this paper we describe the basic mechanisms of nuclear energy and nuclear weapons, and the manufacture of nuclear fuels. Fission and fusion are the two most important nuclear processes for generating nuclear energy and both have been used to make nuclear weapons. The basic physical factors involved in nuclear weapons include neutron multiplication, self－continuity of the fission chain reaction, critical mass and the structure of the nuclear bombs. For hydrogen and neutron bombs we only give brief reviews. As important aspects in nuclear weapon manufactrue, uranium isotope separation and 239Pu production by reactors are reviewed.