Various earlierproposed perturbative approaches to the description of weakly interacting species are considered. It is argued that most of these perturbative approaches, including many of the socalled exchange perturbation theories, exhibit a number of difficulties and limitations, which have not previously been emphasized. From these criticisms positive suggestions as to the nature of a general exchange perturbationlike theory are made. Finally, a candidate for such a theory is outlined. Copyright 1987 John Wiley & Sons, Inc.
