The experimental technique relying on the extraction of kinetic information from the random fluctuations of measured signals is called fluctuation spectroscopy or correlation spectroscopy; it has been recognized as an important biophysical tool. The measurements of fluctuations in the intensity of quasi-elastically scattered light have been used to determine diffusion constants of macromolecules. Voltage and current fluctuations have been used to study the behavior of ionic channels in membranes, and the measurement of concentration fluctuations has been adapted to the study of the kinetics of chemical reactions. The advantages of the fluctuation approach is that the time-dependent behavior of the individual contributor can be determined from the macroscopic signal, that no perturbation of the system is necessary, and that the methods are applicable to the in vivo situation. The distinguishing feature of these approaches is that they monitor spontaneous fluctuations of cross-bridge-related parameters under conditions of thermodynamic equilibrium or steady state. Amplitudes and time dependence of fluctuations are related to the molecular concentrations and the kinetic or diffusive characteristics of cross-bridges, and they connect the fluctuation approach with the conventional relaxation methods.