This document summarizes rumen function and factors that can lead to rumen dysfunction. Proper rumen function requires rumination to break down feed particles and maintain pH, as well as motility to mix contents and absorb fermentation acids. A key regulator of rumen pH is the balance between production and absorption/passage of fermentation acids. Low rumen pH can decrease fiber digestion and passage rates. Risk factors for low pH include low rumen fill, highly fermentable rations, and feeding grains separately from forages. Low pH can damage the rumen epithelium and increase risks of acidosis.