Rumen is part of the forestomach of ruminants and plays a key role in the conversion of feed into metabolites that are absorbed and used by the host. The rumen is also the place of formation of proteins of microbial origin, which represent a source of energy for the host animal. From a functional point of view, ruminants are monogastric at birth as they have an undeveloped forestomach system. Microbial communities in the rumen first show colonization by bacteria, followed by that of methanogenic Archaea and then anaerobic fungi and protozoa. In newborn calves, molecular-based techniques evidenced initial rumen colonisation by facultative anaerobic bacteria, as the phyla Proteobacteria and Firmicutes, with genera Enterococcus and Streptococcus and the species Escherichia coli, followed by Archaea within a few hours after birth. These early colonizers utilize the oxygen available in the rumen, thus creating an anaerobic environment conducive to the growth of rigorous anaerobic communities, including Bifidobacterium and Bacteroides. The strict anaerobic bacterial community, including cellulolytic and proteolytic bacteria, establishes and dominates the rumen microbiome within the first two weeks of life. The entire microbial community allows ruminants to use ligno-cellulosic materials and non-protein nitrogen to produce high-quality food. Importantly, these close anaerobic bacterial communities in the rumen of newborns play an essential role in the development of the mucosal immune system. A healthy rumen leads to healthy ruminants with optimal performance. It is worth highlighting the importance of the microbiome in maintaining the health of cattle and its potential in alleviating disease. This mini-review described the development of the cow microbiome in the rumen, the degradation abilities and influence of the feed on the rumen microbiota, and the microbiota effects on the cow’s immune system and health.