Microarray-based experiments querying tens of thousands of genes, peptides, and other cellular components of varying organisms have evolved over the past several years to become a cornerstone screening method for numerous diseases and conditions, especially those with known genetic etiologies. They are also rapidly becoming a standard tool among researchers investigating reactions within and between hosts and agents relative to shifting environments and environmental conditions, and for quantitatively assessing risk to the health of humans, animals, and plants. The most commonly used microarray technologies for investigating gene expression are described herein, as well as examples of current and future research areas in public health.