Heat shock proteins (HSPs) or stress proteins are produced by prokaryotic and eukaryotic cells in response to a variety of insults. After this original definition, it has become increasingly clear that HSPs can modify the function and destiny of other proteins and thus play an important role in numerous physiological processes. The heat shock response is one of the most universal reactions known and HSPs are among the most conserved molecules in phylogeny. Here Stefan H.E. Kaufmann discusses the role of HSPs in immunity with respect to both their function and their antigenicity.