This chapter briefly investigates collisional effects on plasma-wave echoes. It begins with a treatment of the problems associated with temporal and spatial propagation of small-amplitude waves in plasmas. It considers a situation in which a spatial distribution of potential field is applied to a plasma impulsively at time t = 0; the potential field will evolve subsequently in the plasma. The evolution is described in terms of the dielectric response function; it involves temporal decay due to the Landau damping. The reversible nature of the Vlasov equation is reflected in the microscopic phase evolution of the particle distribution function after the application of a disturbance to the plasma; a macroscopic quantity such as the electric-field potential, on the other hand, would decay through Landau damping. It has been recognized that a macroscopic quantity might then reappear in the plasma if people could reverse the direction of phase evolution of the microscopic elements.