Two distinct but not mutually exclusive types of genetic exchange operate in RNA viruses.  The first, reassortment, occurs only in multipartite viruses and involves swapping one or more of the discrete RNA molecules that make up the segmented viral genome.  Antigenic shift in influenza A virus is an example of this sort of genetic exchange and serves as a good illustration of the potential evolutionary significance of such events.  A second process, recombination, can occur in either segmented or unsegmented viruses when 'donor' nucleotide sequence is introduced into a single, contiguous 'acceptor' RNA molecule to produce a new RNA containing genetic information from more than one source.  In this paper we focus on this type of  genetic exchange.  First, we briefly review current knowledge of RNA virus recombination and describe new methods for detecting its occurrence using gene sequence data.  We then discuss some of the constraints that may shape the variety of RNA virus recombination.