Conventional seismic processing assumes that primaries are our wanted signal and that multiples are noise. This present topic discusses the characteristics of these multiples and how they lead to the selection of the most appropriate attenuation method. We define primaries as signals that have only one downward path, one reflection and one upward path. However, in practice, we frequently use sources and receivers below the surface and this leads to ghost reflections. When these ghost reflections follow the primary closely, within perhaps 20 ms, these are treated as being part of the source pulse or the receiver response and not considered as multiples though technically they are multiples.