Research on combining pro- and prebiotics as synbiotics to enhance human and animal health has accelerated in the past 10 years, including many clinical trials that have assessed a diverse range of synbiotic formulations. In this review, we summarize these studies as well as the commercial applications of synbiotics that are available. In particular, we critically assess the claimed health benefits of synbiotic applications and the ecological and therapeutic factors to consider when designing synbiotics and discuss the implications of these concepts for future research in this fiel Expected final online publication date for the Annual Review of Food Science and Technology Volume 9 is March 25, 2018. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.