Water appears in the atmosphere in a variety of forms, usually referred to by the term hydrometeor, which includes particles as diverse as cloud, rain drops, snowflakes, ice crystals, hail and graupel. Of these, rain, hail, graupel and snow are generally recognised as precipitation. The effects that hydrometeors have on communications systems are dependent both on the system frequency and the type of particle present. At any given instant, of course, more than one type of particle will affect a given link. For example, an earth-space link may often encounter rain over the lower part of the path, and snow at greater heights. We will consider first the various forms of hydrometeor, together with their relevance to radiowave propagation. A brief theoretical framework will then be presented as a background to the model development process and, finally, specific effects of hydrometeors on systems will be considered.