Numerous studies indicate that carbon monoxide (CO) participates in a broader range of processes than any other single molecule, ranging from subcellular to planetary scales. Despite its toxicity to many organisms, a diverse group of bacteria that span multiple phylogenetic lineages metabolize CO. These bacteria are globally distributed and include pathogens, plant symbionts and biogeochemically important lineages in soils and the oceans. New molecular and isolation techniques, as well as genome sequencing, have greatly expanded our knowledge of the diversity of CO oxidizers. Here, we present a newly emerging picture of the distribution, diversity and ecology of aerobic CO-oxidizing bacteria.