Forage utilization is the key distinguishing feature of ruminant agriculture, and the ruminal microbiome is the agent by which these forages are converted to VFA and microbial cell protein that nourish the ruminant host. Culture-independent studies, based on phylogenetic classification of sequences of small-subunit ribosomal RNA molecules, have shown an impressive diversity in species composition of the ruminal microbiome. Because forages are compositionally and structurally more diverse and more complex than are grains and other concentrates, they have the potential to support a greater diversity of microbes within the rumen, and this diversity is extended further when forages are combined with concentrates in a TMR. Although microbiome composition varies across individual animals fed the same diet, certain forage-specific patterns in community composition have emerged. High-forage diets generally increase the relative abundance of phylum Firmicutes relative to phylum Bacteroidetes, consistent with higher ruminal ratios of acetate/propionate expected from known differences in the physiologies of cultured members of these phyla. Worldwide ruminal microbiome census studies have shown that forage-fed ruminants also host elevated populations of uncultured Bacteroidales, Clostridiales and Ruminococcaceae. Subtle differences in forage form may also affect microbiome composition. For example, orchardgrass fed as pasture selectively increases the abundance of genus Butyrivibrio compared with feeding the same forage as hay, and the parallel observed increase in butyrate production may enhance development of ruminal papillae essential for efficient VFA absorption. Bacterial species differ in their affinities for particular plant tissue types, further adding to the complexity of microbial interactions during forage degradation. Several recent studies with cows on mixed rations have shown associations between specific bacterial taxa and important production metrics such as feed efficiency and milk composition, but such studies have not yet been carried out on all-forage diets.