The myxomycetes (plasmodial slime molds or myxogastrids) are a group of fungus-like organisms usually present and sometimes abundant in terrestrial ecosystems. The exact evolutionary affinities of the myxomycetes are still debated, but these organisms constitute a well-defined and homogenous group of approximately 900 species. The reproductive, or spore-producing, stage in the myxomycete life cycle can achieve macroscopic dimensions and be collected and preserved for study in much the same way as the sporocarps of fungi. However, most species of myxomycetes tend to be rather inconspicuous or sporadic in their occurrence and thus not always easy to detect in the field. Moreover, fruiting bodies of most species are relatively ephemeral and do not persist in nature for very long. Myxomycetes also spend a portion of their life cycle as small eukaryotic microorganisms, when their very presence in a given microhabitat can be exceedingly difficult if not impossible to determine. Because of their life history strategy and inconspicuous nature, myxomycetes provide an immense challenge in biodiversity assessments, but studies carried out throughout the world in the context of a major project (entitled “Global Biodiversity of Eumycetozoans”) funded by the National Science Foundation of the United States have provided a considerable body of new data on the assemblages of myxomycetes associated with all types of terrestrial ecosystems. Herein we provide an overview of what is now known about the global distribution and ecology of this group of organisms.