The growth of interest recently in co-evolution of hosts and their parasites arises partly from the hope that a better understanding of the processes involved should provide new insights into disease control in agriculture. Unfortunately, examination of natural systems makes it immediately clear that they are often complex and that the host-parasite interaction may be obscured by many other interactions with different organisms and with physical factors that all affect evolution. For example, though it may be generally true that parasites are deleterious to their hosts in natural systems, this may not always be the case. There is evidence that some parasites are directly beneficial to their hosts: others may be cryptically so. Furthermore, although processes such as density-dependent and frequency-dependent selection are considered to be important in host parasite co-evolution, there is little direct evidence for such effects. The role of gene-for-gene interactions and the effects of “unnecessary” virulence genes are also poorly understood. The principal reason for our lack of understanding of host-parasite co-evolution is that there have been no large-scale analyses of natural ecosystems that have been repeated at regular intervals to follow the dynamics of the host and parasite genotypes under selection.