A review is presented of theoretical and experimental results of an investigation of natural fluctuations in gas and solid-state lasers. All the known theoretical results are obtained by a single method based on the equations of the quasiclassical laser theory. In the calculation of the fluctuations, the equations for the amplitudes and phases of the field are regarded as Langevin equations with suitably introduced fluctuation sources. The sources of the thermal fluctuations of the resonator are determined by the Kallen-Welton formula. The sources of the non-equilibrium fluctuations of the polarization of the working medium are calculated on the basis of the equations for the density-matrix elements. The fluctuations of the amplitudes and phases in linear and ring lasers are considered under arbitrary pump-to-threshold ratios. The natural line width of the laser radiation is calculated. The coupling of opposing waves in a ring laser, due to scattering by the mirrors, is taken into account. The maximum sensitivity of a laser gyroscope is estimated. The results of the theory are compared with the experimental data. A brief description of the status of the problem of calculating the natural fluctuations in lasers and an estimate of the possibilities of the quasiclassical method of calculating fluctuations are given in the conclusion.