The human gut harbors a complex community of bacteria, viruses, fungi, protists, and other microorganisms (collectively termed the microbiome) that impact health and disease. Emerging studies indicate that the gut bacterial microbiome and virome play an important role in healthy infant development. In turn, the composition of the microbiome during development can be influenced by factors such as dietary, environmental, and maternal conditions. As such, the microbiome trajectory during early infancy could be predictors of healthy development. Conversely, adverse early events in life may have consequences later in life. This review focuses on our understanding of the bacterial microbiome and virome during early development, conditions that might influence these processes, and their long-term implications for infant health.