Microbial communities occupy practically all habitats on Earth, from ocean deeps to the human gut, and are pivotal to the ecosystem function. They also hold a vast biotechnological potential to realize functionalities typically beyond the reach of single species, e.g. valorisation of complex resources. Rational design and modulation of communities can thus help addressing outstanding challenges in health and bio-sustainability. Yet, this remains difficult due to the complexity of interspecies interactions. The overarching goal of the ModEM project is to combine mathematical modelling, genetics, omics and laboratory evolution to identify new ways to modulate complex microbial communities. Kefir, a natural milk-fermenting community, and stable assemblies of human gut bacteria are being used as two model systems. The results will have fundamental implications for modulating microbial communities relevant for environment, health and biotechnology.