This document discusses the relationships between human nutrition, the gut microbiome, and the immune system. It argues that understanding how diet influences the gut microbiome and immune system could help address global health problems like malnutrition. The marriage of metagenomic methods to study the gut microbiome and gnotobiotic methods using germ-free animals could provide insights into these relationships and help test hypotheses. Dietary patterns are changing worldwide due to various social and economic factors, and understanding how these changes impact the gut microbiome may provide clues about nutritional status and immune function.