A theory of cold fusion is presented, based on the Bloch theorem. The Bloch functions are used to represent the charged reactants and products of the nuclear fusion reaction in solid-state crystals. The nuclear fusion reaction is treated as a perturbation, the validity of which is shown. Field operator formalism, or quantum field theory, is used to calculate the transition matrix elements. Density of final states is calculated based on the phonon theory. The reaction rate and fusion power output density are calculated by Fermi's golden rule, and from them it is recognized that they look as if they had no reproducibility—unless it is known that they depend on the number of the primitive cells in one crystal, the numbers of both the reactants and products, and the degree of the effectiveness of the Pauli exclusion principle. The triggering mechanism may also have a relation with its dependence on the aforementioned parameters. Three selection rules are derived. One of them is very important and valuable because it suggests that cold fusion is a very clean energy resource; i.e., the radioactivity level of cold fusion is extremely low and safe compared with its output power or the current fission output power. The ratio (f/t) of the production rate of 4He (heat) to that of tritons is derived quantitatively and compared with the observed value. The necessary conditions for cold fusion to occur and continue are given. Quantitative descriptions about nuclear fusion reactions in light (or hydrogen) water electrolysis are also given.