The hologenome concept of evolution posits that animals and their symbiotic microbes are emergent individuals, or holobionts, exhibiting synergistic phenotypes that are subject to evolutionary forces. Its premises are that interactions between animals and their microbes affect the fitness of holobionts, in both beneficial and deleterious ways, and that microbes and their functional genes can persist across animal host generations with fidelity. Covariance between the host genome and the microbiome can thus be maintained and holobiont phenotypes encoded by the microbiome can be subject to selection and drift within holobiont populations. Animal behavior researchers have historically underappreciated the beneficial effects of symbiotic microbes on animals’ behavioral phenotypes, but this is changing. Symbiotic microbes can protect their host animals from predators, increase their foraging efficiencies and reproductive outputs, and mediate their chemical communication systems. The objectives of this chapter are to introduce the hologenome concept of evolution and, within the framework of the concept’s premises, to present highlights of the current understanding of how symbiotic microbes contribute to animals’ behavioral phenotypes and how animals facilitate transmission of beneficial microbes to their offspring and kin. The chapter concludes with a discussion of how behavioral ecologists, in particular, are well-positioned to evaluate the explanatory value of host-microbial evolutionary models, such as the hologenome concept.