Gene expression microarrays are high throughput technologies that can simultaneously measure the expression levels of most known genes in the human genome within a biological sample. The study of gene expression has revealed new understanding into the biological complexities of the cell and can impact the field of medicine by providing new insights into disease. Examining gene expression in samples from patients with pulmonary disease can elucidate molecular mechanisms responsible for disease pathogenesis or uncover novel molecular subtypes within a disease. Gene expression signatures of disease pathogenesis can further be used to suggest novel therapeutic compounds. Biomarkers can be developed from gene expression data that can aid clinicians in diagnosing disease or can guide clinicians in tailoring therapeutic strategies to individual patients. To demonstrate the applications of gene expression microarray technology, we will review several studies in pulmonary disease that utilize gene expression profiling techniques to gain biological insights into disease or to develop clinically relevant biomarkers for disease management.