At birth, a dairy calf’s rumen is not yet developed physically or metabolically. Thus, the calf functions as a monogastric animal until 4 to 8 weeks of life. In order for a calf to transition to a ruminant, the rumen must be able to support fermentation. This development of the rumen is necessary for successful weaning and is primarily affected by dietary change. The rumen changes physically by increasing mass and growth of papillae. At birth, a calf’s rumen is only 25% of the stomach capacity, but a mature cow’s rumen can compose up to 80% of its stomach. The physical stimulation of feed in the rumen can cause increases in rumen weight and muscular development. However, physical bulk alone has not been proven to promote papillary development. When calves drink milk they stimulate the opening of the esophageal groove. This closure keeps milk from entering the reticulo-rumen and prevents fermentation in the rumen. Calves should be given free choice water to produce the liquid environment needed for microbial growth. Besides liquid and muscular action, the ingredients needed in rumen development consist of bacteria, absorptive ability of the tissue and substrate. A newborn calf has a sterile rumen, but by 2 d after birth microbes are colonized. These microbes influence growth and development. The earlier dry feed and forage is given to calves, the earlier microbial development occurs. Higher rumen metabolic activity and increased Volatile Fatty Acid concentrations are a direct reflection of the nutrient substrate required for rumen microbes. The fermentation end products of butyrate and propionate are responsible for the growth of ruminal papillae. In conclusion, early consumption of dry feed develops the calf rumen physically and metabolically by promoting growth of rumen epithelium and mobility.