Although it has been more than one hundred years since the first publication on the milks of whales and dolphins (Order Cetacea), information on lactation in these species is scattered and fragmentary. Yet the immense size of some cetaceans, and the recent evidence that another group of marine mammals, the true seals, have remarkable rates of secretion of milk fat and energy, make this group of great comparative interest. In this paper information on lactation patterns, milk composition and lactation performance is reviewed. Two very different patterns are evident. Many of the baleen whales (Suborder Mysticeti) have relatively brief lactations (5-7 months) during which they fast or eat relatively little. At mid-lactation they produce milks relatively low in water (40-53%), high in fat (30-50%), and moderately high in protein (9-15%) and ash (1.2-2.1%). From mammary gland weights and postnatal growth rates, it is predicted that their energy outputs in milk are exceptional, reaching on the order of 4000 MJ/ d in the blue whale. This is possible because pregnant females migrate to feeding grounds where they can ingest and deposit great amounts of energy, building up blubber stores prior to parturition. On the other hand, the toothed whales and dolphins (Suborder Odontoceti) have much more extensive lactations typically lasting 1-3 years, during which the mothers feed. At mid-lactation their milks appear to be higher in water (60-77%) and lower in fat (10-30%) and ash (0.6-1.1%), with similar levels of protein (8-11%). At least some odontocetes resemble primates in terms of low predicted rates of energy output and a long period of dependency of the young. However, these hypotheses are based on small numbers of samples for a relatively small number of species. Much of the available data on milk composition is of rather poor quality; for example, it is not possible to determine if milk composition changes over the course of lactation among odontocetes. Additional research on cetacean mammary glands and their secretions is needed to understand the reproductive strategies of these fascinating animals.