The Waves experiment on the Wind satellite has been used to survey the occurrence of nearly monochromatic waves in the Earth's tail region. Several hundred examples of monochromatic waves were observed during excursions into the tail, including 218 examples during a 4-day passage of the Wind satellite through the geotail at -90 RE. We present an investigation of the characteristics of these waves and of the environment in which they are found. The waves are electrostatic, and the distribution of the angle E to B shows peaks at 0¿ and 90¿. The waves near 90¿ sometimes show electron cyclotron harmonics, but these are very rare in the waves near 0¿. It seems that the waves occur most commonly in or near the plasma sheet boundary layer, and it is probable that they are convected downstream from reconnecting regions.