The rumen contains a complex microbial ecosystem populated by prokaryotes (bacteria and archaea) and eukaryotes (protozoa and fungi) highly adapted to the fermentation of solely herbivorous feeds, and to survival in a gut environment. In the rumen there are also dense viral populations which both co-exist with and predate on, the rumen microbes. Similarly to other microbial ecosystems, the microbes of the rumen have the capacity for horizontal gene transfer, which to date is largely unstudied. Gene transfer in the rumen is expected to be facilitated by viruses and extrachromosomal elements (for example plasmids), which collectively contribute to the rumen mobilome. This chapter explores the rumen mobilome, reviewing current knowledge and technologies which have driven research efforts, and detailing the possible impacts of the mobilome on rumen function.