Disease control exerts evolutionary pressures that can lead to the evolutio
n of resistance. This has been seen in a spectacular fashion in the evoluti
on of resistance to antibiotics, anti-virals and anti-parasitics. Despite i
ntense (and often successful) attempts to control infectious diseases throu
gh vaccination, there is still rather little evidence of the emergence of s
trains of pathogen resistant to vaccines. This chapter asks why this should
be so and what are the exceptions indicating that the evolution of vaccine
resistance, though currently rare, is a possibility that should be planned
for.