Abstract: Across insects and arthropods, associations with intracellular microbial endosymbionts are common. While endosymbionts sometimes form charismatic obligate relationships with their hosts, most associations in nature are facultative and polymorphic from the host perspective. I will discuss the acquisition, spread, and maintenance of extraordinarily charismatic Wolbachia bacteria that now occur in about half of all insect species, making them the most common endosymbionts in nature. Alternative calibrations suggest that within hundreds of thousands of years or less Wolbachia have invaded hosts diverged hundreds of millions of years. Once acquired, it seems most plausible that Wolbachia initially spread within host species by positively influencing host fitness, although the specific effects are challenging to identify. I will present evidence for Wolbachia effects on host fecundity, viability, and fertility that depend on genomic interactions and plausibly contribute to spread. Following initial spread, equilibrium Wolbachia frequencies are governed by their vertical transmission rates and effects on host reproduction. These include Wolbachia-induced cytoplasmic incompatibility (CI) that favors spread to higher frequencies within populations and increases the likelihood of horizontal transfer across species. Mathematical models indicate that selection should not preserve CI, yet our tests for selection support the preservation of a CI-causing factor, potentially because of its pleiotropic effects. Empirical parameter estimates and mathematical modeling indicate that genomic and environmental interactions contribute to Wolbachia prevalence, including in transinfected vector systems where Wolbachia from Drosophila block pathogens and protect millions of individuals from disease. I will discuss critical gaps in our understanding that currently limit inferences.