The availability of iron plays a critical role in host-parasite relations. Microbial pathogens cannot replicate without acquiring iron from their vertebrate hosts, and several strategies of microbial iron acquisition have evolved. Vertebrate animals have developed elaborate strategies to withhold iron from invading pathogens while retaining their own access to the metal. This review focuses on recent data from studies of both microbial iron-acquisition mechanisms and the vertebrate host's iron-withholding defense system components.