Various types of echo oscillations in plasmas are discussed. Both unbounded plasmas (gaskinetic plasmas and plasmas in external fields) and bounded plasma systems are considered. Several practical applications of echo effects in radio wave propagation and plasma diagnostics are discussed. The second- and third- order echo oscillations in equilibrium isotropic and magnetized plasmas and in semiconductor plasmas are discussed. A charged-particle beam in a non-equilibrium plasma can lead to an amplification (over time) or a growth (over space) of echo oscillations. A ballistic theory is offered for echos in plasmas. This theory can describe echos in the case of large-amplitude external perturbations, under conditions such that the method of successive approximations cannot be used. The ballistic theory can also describe echo saturation. Echo phenomena in inhomogeneous plasmas are discussed. Section 2 reports a study of echos in bounded plasmas. In this case, additional echo phenomena result from the reflection of charged particles from the plasma boundaries and from the existence of surface waves. Research on echos in plasma slabs and metal films is reviewed. Echo effects can lead to an anomalous transmission of electromagnetic waves through a plasma slab which is opaque (in the linear approximation) to the fields of these waves. Section 3 reviews the basic experimental data on echos in the cyclotron, ion, and plasma zones and research on collisions and microscopic turbulence in plasmas by means of echos. The experimental data available are compared with the theoretical results.