Advisor: Frederic D. Bushman, PhD. The human body contains large numbers of viral particles (over 1012 per person), largely bacteriophage, but little is known of how these viral communities influence human health and disease. To study the viruses of the human gut (the so-called gut `virome') during a known environmental perturbation we collected stool samples from healthy individuals participating in a controlled diet study. Viral DNA was purified and deep-sequenced using 454 and Illumina technologies, yielding over 48 billion bases of viral sequence spread across 28 samples from 12 healthy individuals. Computational analysis of this unprecedentedly large database of viral sequences allowed us to characterize these communities on a genomic level. We found that the vast majority of viruses from the human gut were novel species of bacteriophage, and that only 1 of these 12 individuals contained a known eukaryotic DNA virus. Temporal changes in these viral communities were correlated with experimental manipulation of diet, and parallel deep sequencing of gut bacteria revealed co-variation between bacterial and viral communities, supporting the hypothesis of linked reproduction between these two groups. A large proportion of viral contigs have markers of temperate lifestyle, indicating that there is a significant role of lysogeny in the gut microbiome. Analysis of genetically variable elements within these viral genomes revealed novel classes of diversity-generating retroelements targeting immunoglobulin-superfamily proteins, suggesting a surprising example of convergent evolution with the vertebrate immune system. Optimization of assembly algorithms for these samples improved the recovery of complete and partial genome sequences. While the assembled genomes were highly dissimilar on the nucleotide level, analysis of syntenic protein-coding sequences revealed conserved gene cassettes that display an inferred structural and functional conservation despite a high degree of nucleotide substitution. Through high-throughput shotgun sequencing of viral DNA, we found that the healthy human gut contains a wide variety of extremely diverse bacteriophages encoding novel and unexpected functions. This work sets the stage for thorough genomic analysis of complex viral communities, and presents the intriguing problem of how this immense pool of genetic diversity has evolved and persisted.