Microbial communities constitute the most ancient, numerous, and complex components of natural ecosystems. The developments in high-throughput techniques, provide microbial ecologists new opportunities to integrate ecological theories into their research and to tackle one central questions of ecology: what are the mechanisms driving community assembly? Consequently, classic ecological debates about niche vs. neutral theories or deterministic vs. stochastic processes now stand on the microbial ground. Recently, microbial ecologists went beyond borrowing concepts and approaches from general ecological research but rather provided substantial developments. Using various established and novel approaches, they mined deeper into the various mechanisms and ecological processes shaping communities, and, were able to delineate the contribution of environment filtering, species interactions, historical contingencies, priority effects, dispersal, diversification and ecological drift on microbial community assembly. These works revealed intriguing patterns at various spatial and temporal scales in different ecosystems, including soil, surface, subsurface and urban waters, laboratory systems, along with host-associated communities in plants and animals. Meanwhile, many questions and challenges have emerged. For instance, how to reconcile diverse conceptual frameworks and terminologies towards a coherent picture of microbial assembly mechanisms? How can different approaches be validated and evaluated? How to build better approaches by including different facets of microbial biodiversity (taxonomic, phylogenetic, and functional)? What are the similarities and discrepancies between the assembly mechanisms shaping macro- and micro-organisms communities? We are still many steps away from incorporating microbial ecological information into the large pool of ecological theories and identifying potential general rules of microbial assembly across spatial or temporal scales in diverse ecosystems is one of them. Hence, the goal of this oral session is to rally ecologists, microbiologists, and bioinformaticians with broad backgrounds to discuss the following topics: (1) theories and conceptual frameworks of microbial community assembly; (2) approaches based on various biodiversity dimensions to infer microbial assembly mechanisms; (3) experimental research on microbial assembly theories; (4) major ecological processes shaping microbial community structure in natural and artificial environments; (5) drivers and rules of assembly mechanism variation across spatial and temporal scales. This forum is the opportunity to examine the current state of research on these topics and paint a picture of the possibilities generated by a better understanding of microbial assembly mechanisms, in a world increasingly realized to be standing on a microbial foundation.