Human microbiome studies have exploded in number in recent years with the advent of high-throughput sequencing methods. The dramatic increase in sensitivity facilitated by deep-sequencing capability has led to hundreds of studies characterizing microbial communities in multiple human tissues, as well as the relationships between the composition of these communities and disease. The most-studied body sites in this regard have been the components of the gastrointestinal (GI) tract, partially driven by stool sample sequencing. Stool can be utilized as a non-invasive way to query the human gut. Gut microbiome studies have revealed important connections with many diseases, including cancer. More recent works have begun to investigate microflora at cancer sites themselves in a variety of solid tumor types, finding microbial "signatures" that can distinguish tumors from normal tissues of the same type. These signatures are also distinct between tumor types and are detectable, to some degree, in circulation, raising the possibility of a blood-based test for cancer based on microbial DNA.