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: Bioluminescence imaging (BLI) of bacteria was primarily designed to permit real-time, sensitive, and noninvasive monitoring of the progression of infection in live animals. Generally, BLI relies on the construction of bacterial strains that possess the lux operon. The lux operon is composed of a set of genes that encode the luciferase enzyme and its cognate substrate, which interact to produce light-a phenomenon that is referred to as bioluminescence. Bioluminescence emitted by the bacteria can then be detected and imaged within a living host using sensitive charge-coupled device (CCD) cameras. In comparison to traditional host-pathogen studies, BLI offers the opportunity for extended monitoring of infected animals without resorting to euthanasia and extensive tissue processing at each time point. Therefore, BLI can reduce the number of animals required to generate meaningful data, while significantly contributing to the understanding of pathogenesis in the host and, subsequently, the development and evaluation of adequate vaccines and therapeutics. BLI is also useful in characterizing the interactions of pathogens with plants and the para-host environment. In this chapter, we demonstrate the broad application of BLI for studying bacterial pathogens in different niches. Furthermore, we will specifically focus on the use of BLI to characterize the following: (1) the pathogenesis of Brucella melitensis in mice (animal host), and (2) the progression of infection of Clavibacter michiganensis subsp. michiganensis in tomatoes (plant host). These studies will provide an overview of the wide potential of BLI and its role in enhancing the study of unique-and sometimes difficult-to-characterize-bacterial pathogens.