Firstly, the development and application of the coupled-mode theory in microwaves and its extension to fiber optics in the early years are briefly reviewed, and it is well known that coupling among modes is described mathematically by the coupled-mode equations which are linear differential equation group of the first order. Then, it is noted that the coupled-mode equations are rigorously transformed from the Maxwell equations along with a perturbed boundary condition, the approximation methods are used when solving these equations and the order of the approximation is given. The coupled-mode method is attributed to its simplicity in principle, intuitiveness in physics and capacity of simulating complete transfer of power. Finally, a variety of applications of coupled-mode theory to fiber optics, including modeling and simulation are shortly introduced. Misuses of coupled-mode theory are also commented.