Host-microbiota interactions involving inflammatory and metabolic pathways have been linked to the pathogenesis of multiple immune-mediated diseases and metabolic conditions like diabetes and obesity. Accumulating evidence suggests that alterations in the gut microbiome could play a role in cardiovascular disease. This review focuses on recent advances in our understanding of the interplay between diet, gut microbiota and cardiovascular disease, with emphasis on heart failure and coronary artery disease. Whereas much of the literature has focused on the circulating levels of the diet- and microbiota-dependent metabolite trimethylamine-N-oxide (TMAO), several recent sequencing-based studies have demonstrated compositional and functional alterations in the gut microbiomes in both diseases. Some microbiota characteristics are consistent across several study cohorts, such as a decreased abundance of microbes with capacity for producing butyrate. However, the published gut microbiota studies generally lack essential covariates like diet and clinical data, are too small to capture the substantial variation in the gut microbiome, and lack parallel plasma samples, limiting the ability to translate the functional capacity of the gut microbiomes to actual function reflected by circulating microbiota-related metabolites. This review attempts to give directions for future studies in order to demonstrate clinical utility of the gut-heart axis.