Unicellulars represent ~70% of extant eukaryotic diversity and are responsible for the production of half of all oxygen on Earth. Major challenges in the field are underpinned by this staggering diversity of this domain of life, much of which remains uncultured. Major progress has been achieved in understanding genomes/transcriptomes/proteomes, evolutionary relantionships and diversity of selected representatives. Still, for the vast majority of microbial eukaryotes, be they terrestrial or aquatic, we know nothing about their interactons with other living entities. Therefore, with this series of EMBO workshops, which was so far extremely helpful in propelling comparative genomics and diversity of microbial eukaryotes, we will now pay more attendtion to microbial interactions, as they truly underpin the survival of all life. In order to understand microbial ecosystems, we need to focus on interactions, ranging from ephermeral to permanent, and their evolutionary, physiological, and ecological implications. These topics will be combined with bringing forward the latest developments in the studies of the last eukaryotic common ancestor and non-traditional model organisms and knowledge of their cellular biology – especially as it connects to interactions with the external world. Inspired by the currect pandemics, we will also pay attention to viruses affecting this domain of life.