The Barcelona Debates on the Human Microbiome celebrates this year its 9th edition and has become a well stablished meeting of reference, engaging the most prestigious speakers and gathering a wide audience. It brings together leading-edge scientists working on different levels and aspects in the understanding of the complex interactions between human cells and the microorganisms that live in the human body, such as professionals of the medicine and health sciences, biosciences and mathematics, to share new insights in this wide field and attempt to identify promising directions for future research.