Dietary shifts disturb microbiota and fermentation of the rumen, and varying amounts of time are required for them to re-stabilize. Shifts from high-forage to high-grain diets have been studied for over 60 years. During these shifts, abundance of total and cultured bacteria generally increases and then stabilizes. For some shifts in some studies, stabilization was complete within 24 h, but in others, it was incomplete even after 3 wk. Specific groups of bacteria (cellulolytics, amylolytics, lactate-utilizers) and protozoa also require varying amounts of time to stabilize. The time required for methanogens and fungi to stabilize has seldom been studied. Stabilization of fermentation (pH, short-chain fatty acid concentration) does not necessarily coincide with stabilization of the microbiota. For diets with very high (>70%) grain, abundance of protozoa and cultured bacteria vacillates greatly and stabilizes slowly, if at all. More recent studies have employed culture-independent methods, such as qPCR and 16S rDNA sequencing, to investigate taxonomic composition of bacteria and methanogens during dietary shifts. These studies confirm that taxonomic composition does change during dietary shifts, but more frequent sampling is required to resolve at what exact point it stabilizes. Other culture-independent methods, such as whole-genome or transcriptome sequencing, need to be deployed to resolve changes in microbial function, not just taxonomic composition. Changes in microbial communities, whether assessed by culture-independent or dependent methods, need to be better related to variables of interest in feeding trials (e.g., diet intake and digestibility).