The thermal, acoustic and dielectric properties of glasses at low temperature are attributed to two level defects either elastic or electric. These obey to the same description as that of the spin 1/2 particle. Amongst the various methods of probing such defects, the technique of echoes has recently proven to be most efficient. A review is made of the different types of echoes observed in glasses, with emphasis given to electric dipolar echoes. From the results of these measurements, values are obtained for the density of states, the longitudinal and transverse components of the dipole moment, the coupling to the strain field, also the temperature behaviour of the longitudinal (T1) and transverse (T2) relaxation times are measured