The obligate intracellular bacterial agent of human Q fever, Coxiella burnetii, has a remarkable ability to persist in the extracellular environment. It replicates only when phagocytosed and delivered to the phagolysosome, where it resists degradation. Different morphological forms of the bacterium have different resistance properties and appear to be stages of a developmental cycle. Despite the lack of genetic systems, the molecular events surrounding C. burnetii development are now being unraveled.