The behavior of dielectric glasses at low temperatures when acted on by electric and acoustic fields is discussed. The anomalous low-temperature behavior of the heat conductivity and the heat capacity is described. A phenomenological model of two-level systems in glasses arising from tunneling transitions is presented. Results are given of the experimental and theoretical studies of the phenomena of saturation, induced transparency of the medium by application of acoustic and electric pulses, and ``hole-burning'' in an inhomogeneously broadened line. Considerable attention is paid to discussing the experimental results on generation of coherent responses of echo signals. In order to describe the formation of phonon-echo signals in glasses, the concept is employed of spectral diffusion, which consists of a dynamic alteration of the resonance frequency. We stress both the aspects of the properties that are common with the phenomena of magnetic resonance, and the differences that arise owing to the broad spectrum of the two-level systems.