频率定理（Kalman－Yakubovich引理）是控制理论中的重要结果之一．它给出了Lur’e方程和Riccati方程可解的一个充分必要条件．本文简要叙述了频率定理建立和发展的历史，描述了它在非线性系统绝对稳定性理论，线性二次型最优控制和自适应控制理论中的部分应用． The frequency theorem (Kalman-Yakubovich lemma) is one of the important results in control theory. It gives a sufficient and necessary condition for the solvability of Lur’e and Riccati equations. This paper briefly describes the history of the establishment and development of frequency theorem, and describes its application in the theory of absolute stability, linear quadratic optimal control and adaptive control of nonlinear systems.