Our collective knowledge of calf nutrition has evolved over the past 100+ years, but there are still areas of improvement that merit further scientific inquiry. The work described herein explored different aspects of calf nutrition with a central focus on rumen growth and development. The first study performed used 8 Holstein bull calves to determine if calf starters differing in starch and neutral detergent fiber (NDF) content would affect calf growth, intake, rumen metabolites, blood metabolites, and gross rumen measurements when fed along with milk replacer (MR). The experiment used completely pelleted calf starters consisting of ground and pelleted barley, wheat, and corn grains. Besides the high-starch starter resulting in lower rumen pH, the hypothesis that completely pelleted calf starter diets differing in NDF and starch level would alter intake, growth, rumen metabolism, and rumen measurements was not supported. However, calves fed the high-NDF starter were $5.71 less expensive per calf to raise. Findings suggest a form of feed effect in today's calf starter diets that might be of physiological and economic importance. The second study tested custom-built rumen infusion, sampling, and evacuation devices. The main objectives were to build and confirm the successful use of the devices in one Holstein bull calf at 62 days of age, which determined a liquid passage rate out of the rumen at 40.2% of ruminal fluid/h. The third and final study examined the effects of form of diet (MR only, n = 5; MR and starter, n = 6) on rumen growth and development. More specifically, isocaloric and isonitrogenous diets were fed to neonatal and ruminally cannulated Holstein calves for 6 week. The hypothesis of MR and starter calves having altered gross rumen measurements, epithelial stem and progenitor cell number, and epithelial proliferation status was supported, but hypothesized changes in volatile fatty acid (VFA) transporter abundance and VFA absorption rate were not supported. These results indicate that form of diet, even one that promotes rumen growth, does not equate to enhanced ability to absorb VFA, but there is an effect on rumen stem and progenitor cells as well as epithelial proliferation. / PHD / Understanding the fundamental aspects of calf nutrition is of vital importance for enhancing efficiency of calf growth. Calves are the future of any dairy herd and to perform their best, calves should be managed and fed in ways that ensure efficient growth. Research in these related areas stem back to the 1890s. Questions examined in this body of work mainly focused on dietary impacts on rumen growth and development in young calves. The first trial investigated effects of nutrient composition of pelleted calf starter diets on growth. It was discovered that form of diet, rather than composition, may be more important for causing changes in rumen growth and development. The second experiment resulted in development of a rumen sampling and infusion device to refine calf growth research. The third research trial tested the importance of calf starter on rumen growth and absorptive function. Results indicate that presumptive stem and progenitor cells within the rumen epidermis are identifiable by their ability to retain labeled DNA long-term and change proliferative status in response to diet. Absorptive function was not influenced by calf diet. Starter was important for rumen growth but not its function. Overall this work contributes to the enhancement of growth efficiency in dairy calves.