Gastrointestinal (GI) microbes have important roles in the nutritional, immunological, and physiologic processes of the host. Traditional cultivation techniques have revealed bacterial density ranges from 104 to 105 colony forming units (CFU)/g in the stomach, from 105 to 107 CFU/g in the small intestine, and from 109 to 1011 CFU/g in the colon of healthy dogs. As a small number of bacterial species can be grown and studied in culture, however, progress was limited until the recent emergence of DNA-based techniques. In recent years, DNA sequencing technology and bioinformatics have allowed for better phylogenetic and functional/metabolic characterization of the canine gut microbiome. Predominant phyla include Firmicutes, Bacteroidetes, Fusobacteria, Proteobacteria, and Actinobacteria. Studies using 16S ribosomal RNA (rRNA) gene pyrosequencing have demonstrated spatial differences along the GI tract and among microbes adhered to the GI mucosa compared to those in intestinal contents or feces. Similar to humans, GI microbiome dysbiosis is common in canine GI diseases such as chronic diarrhea and inflammatory bowel diseases. DNA-based assays have also identified key pathogens contributing to such conditions, including various Clostridium, Campylobacter, Salmonella, and Escherichia spp. Moreover, nutritionists have applied DNA-based techniques to study the effects of dietary interventions such as dietary fiber, prebiotics, and probiotics on the canine GI microbiome and associated health indices. Despite recent advances in the field, the canine GI microbiome is far from being fully characterized and a deeper characterization of the phylogenetic and functional/metabolic capacity of the GI microbiome in health and disease is needed. This paper provides an overview of recent studies performed to characterize the canine GI microbiome.