The comfort movements of Anatidae are described in detail on the basis of observations on 114 species including representatives of all tribes except the Torrent Ducks (Merganetta). A fundamental distinction is made between the occurrence of comfort movements in (1) basic situations (where they are apparently serving their original functions, mostly in caring for the body surface) and (2) secondary situations (where the movements appear irrelevant and unlikely to be serving their original functions). Under ''basic situations'', the form of each movement in the Mallard is described, and attention is drawn to variations occurring in other species. Note is made of the situations in which the comfort movement is performed, and its probable function is discussed. Under ''secondary situations'', an attempt is made to distinguish between instances in which the movement is serving as a social signal and those in which it is not. Comfort movements are classified into seven broad categories : shaking movements, stretching movements, cleaning movements, oiling preening, nibbling preening, washing, and bathing. Most of the nine shaking movements probably function to remove water or foreign material from various parts of the body. The frequency of wing-flapping in White-fronted Geese increases in response to rain and dense fog. Vigorous wing-shaking occurs during oiling and it may have a special function in relation to this activity. Wing-shuffling and tail-fanning are performed after bathing and probably help to dry the feathers. There are three stretching movements: wing-and-leg-stretch, both-wings-stretch, and jaw-stretch. They are all associated with periods of rest; the wing stretches occur during preening before sleeping as well as after awakening. Scratching is by the ''direct'' method, and appears to be elicited by an irritation on the head. An ''extended'' type of scratching is associated with bathing, and probably helps to clean the plumage. Scratching is thought to play no part in the distribution of oil to the head plumage. A movement which is probably scratching occurs frequently during flight in the Green-winged Teal; this is the only instance of an aerial comfort movement which has been seen repeatedly in any species of waterfowl. Oiling follows bathing, but it also occurs at other times. Only the most aquatic groups (e.g. Aythya, Bucephala, Oxyura) oil frequently while swimming. Many oiling preens last only for a few minutes; others merge into nibbling preening which may continue for about 30 minutes. In several species at least, oiling is not performed on the nest during the incubation period, but it occurs repeatedly when hatching begins. This behaviour is released by stimuli from the ducklings, and it apparently serves to distribute oil to the ducklings. Occasional nibbling preening movements are probably responses to irritations on the body surface. The causation of nibbling with bill-dipping is uncertain; it may help to clean the plumage or skin. Social preening occurs in a number of tropical waterfowl; it is especially developed in several species of Dendrocygna. It probably supplements scratching as a method of grooming the head feathers. Three bathing movements are distinguished - head-dipping, somersaulting, and wing-thrashing. Dashing-and-diving consists of escape patterns which have come to be closely associated with high intensity bathing. Mallard ducklings perform most comfort movements within the first few days of life. High intensity bathing movements (somersaulting, wing-thrashing, dashing-and-diving) were not seen until the ducklings were 13-14 days old. Almost all comfort movements are of universal occurrence in basic situations in the family. The few specific variations represent adaptations to different ways of life, and are of very limited use as indicators of taxonomic relationships. The causation of comfort movements in basic situations is discussed, but no firm conclusions can be drawn in the absence of experimental evidence. Bathing, shaking, wing-flapping, and a few other comfort movements occur after alighting, after a disturbance, after copulation, and after hostile encounters. These are considered as secondary situations; the comfort movements are thought to be caused, at least partly, by the social tendencies which have been strongly activated. Preening movements occur before changing position on the nest during incubation. Some of these may be responses to irritation. Some function in loosening down from the breast. Many comfort movements occur before flight. Some are repeated with a high frequency and are thought to function as social signals indicating intention to fly, e.g. Head-shaking in many tribes, bathing in certain Mergini. Comfort movements have provided an important source of male pre-copulatory signals, especially in the Mergini. Some have a ritualized form, but most are very similar to the normal comfort movements. Within the Mergini, there are many specific variations in the displays which occur and in the way they are combined in sequences. These displays provide useful taxonomic characters in the tribe. The Swimming-shake (in courting parties) and Preen-behind-wing are widely distributed displays in the duck tribes. There is much variation in their frequency and importance from species to species. Five ritualized Preening movements are distinguished. A correlation between preening displays and conspicuous plumage patterns is not always apparent. Both ritualized and unritualized comfort movements are commonly associated with pair-formation activities and hostile encounters. Displays may have evolved from both ''irrelevant'' and ''relevant'' comfort movements. There is much evidence of the presence of conflicts between antagonistic tendencies in the situations in question (e.g. before flight, before copulation, during pair-formation). Some ritualized comfort movements (shakes, bathing, preening movements) occur in situations where normal counterparts would be expected. Certain nibbling preening movements and several shaking movements occur as displays in many species, but ritualized oiling actions are rare. Some comfort movements (e.g. bill-cleaning, wing-and-leg-stretch, shoulder-rubbing) are used by only a few species. The high intensity bathing movements (wing-thrashing and somersaulting) are not known to occur as displays.