Excessive fermentation in the rumen as a result of energy-rich diets decreases ruminal pH and leads to a digestive disorder called ruminal acidosis. This chapter examines one kind of acidosis, subacute rumen acidosis (SARA), and includes a detailed case study on SARA risk in the post-partum phase of the transition period. The chapter comprehensively analyses SARA and steps that can be taken to minimize it, and looks ahead to likely future trends of research in this area.