Background: Anthocyanins are a class of flavonoids responsible for the red-purple pigmentation of foods such as strawberries, bilberries and black rice. Numerous studies report beneficial physiological effects linked to anthocyanin consumption. But, due to their rapid degradation following consumption, bioavailability of anthocyanins is low, and any protective effects are likely to be attributed to their metabolites [1]. However, much remains to be understood regarding the plethora of metabolites generated and the role of the gut microbiota in this. Objective: To determine the metabolites produced from cyanidin- and delphinidin-type anthocyanins, highlighting inter-individual variation in metabolite profiles, differences in the metabolites from these two anthocyanins, and their relationship with the gut microbiota. Approach and Study Design: In a placebo-controlled randomised crossover trial (performed in compliance with the guidelines laid down in the Declaration of Helsinki), 52 volunteers consumed capsules of bilberry (mainly delphinidin-type) or black rice (mainly cyanidin-type) anthocyanins for 28 days. Urine samples were collected pre- and post-intervention, subjected to solid-phase extraction, and analysed using UHPLC-MS/MS.  For a subset of participants (n = 24) faecal samples were preserved as glycerol stocks and used to establish the real-time anthocyanin metabolising capacity of the microbiota using an in-house method (Shehata, Day-Walsh & Kroon, unpublished).   Results: For the first time, 5-hydroxyferulic acid was identified as a bilberry anthocyanin metabolite. Additionally, several other phenolic metabolites were identified that were not reported in a study of human metabolism of penta[13C]-cyanidin-3-glucoside [2]. We observed catechol and its phase 2 conjugates to be major metabolites of both cyanidin- and delphinidin-type anthocyanins. Furthermore, we have provided evidence for the role of the gut microbiota in the production of key in vivo metabolites, and have shown clear correlations between in vitro fermentation and in vivo metabolism. Conclusion and Future Work: We observe specific differences in the metabolite profiles derived from cyanidin- and delphinidin-type anthocyanins, along with considerable inter-individual variation in metabolism. For the first time we are also able to compare in vivo urinary metabolites with those produced in an in vitro gut fermentation model, highlighting the critical role of the gut microbiota in anthocyanin metabolism, and consequently the physiological effects of an anthocyanin rich diet. An important point of future work will be to elucidate any physiological bioactivity of these microbial metabolites.