The human GI tract is inhabited by an incredibly complex and abundant microbiota, whose composition is dependent on a variety of factors. The gut microbiota has an influence in the morphological, immunological and nutritional functions of the digestive tract and may be involved in many diseases. This article proposes the rationale behind conducting in vitro diagnostics (IVDs) of the human microbiota, as well as outlining the conceptual and technical difficulties involved in IVD testing. The molecular methods that can be used according to whether the IVD tools are employed to study one individual constituent species or to determine the microbiota as a whole will also be described. In the latter case, these technologies include high-throughput sequencing for metagenomics and DNA microarrays, which can now be efficiently used to study gut ecology and are believed to represent the future of standardized diagnostics.