The direct effects of rain on birds is a subject to which no more than passing attention has yet been given. This review is an attempt to summarise the scattered information in the literature and to draw some general conclusions. It is concerned particularly with wetting and its consequences, and with the behavioural responses of the birds.Nye (1964) showed that, if the insulating layer of air trapped under and between the feathers is replaced by water, which is a greater conductor of heat, the wet bird will suffer much larger heat loss, for which it has to compensate physiologically if its body temperature is to remain constant. Heat will also be lost through evaporation of water from the wet plumage. This extra heat loss must be made good by the production of heat from internal reserves. If these are depleted, or if cooling is rapid, death from hypothermia (lowered body temperature) may result. It must therefore be of considerable survival value for plumage to withstand rain, especially when the weather is cold and food is scarce. The questions which need to be answered are: (1) Are some species more prone than others to wetting? (2) To what plumage characters is variability in rain resistance due? (3) What are the consequences of wetting? (4) Are some species more resistant than others to loss of heat after wetting and, if so, how? This review is not, however, concerned with (2), a neglected and complex subject which needs a review to itself, other