This document discusses protein arrays as tools for studying host-pathogen interactions. Protein arrays are similar to DNA microarrays in that proteins or antibodies are attached to a slide to profile protein mixtures, measure binding affinities, and study protein expression levels. They can be used to monitor disease states and for clinical diagnostics. The main types of protein arrays discussed are analytical arrays, functional arrays, and reverse phase arrays. Protein arrays provide a powerful tool for proteomics, biomarker discovery, and analyzing infection responses by profiling antibody and protein signatures.