The oral cavity is the opening of the gastrointestinal tract which harbors a complex microbial community. The unique anatomic location demands that, to colonize and persist in this particular niche, the oral microorganisms need to be able to attach to the available tooth, mucosal, or even artificial surfaces to form biofilms, a surface-attached microbial community composed of one or often diverse microbial species and enclosed in self-produced polymers. In the past decade, facilitated by great technological advances, we have achieved an unprecedented understanding of the complex structure and functions of the oral microbiome. We now know that the oral cavity not only harbors enormously diverse bacterial species but also is home to a multitude of yet-to-be-cultured ultra-small bacteria belonging to the newly classified “Candidate Phyla Radiation” (CPR) group, as well as Archaea, fungi and viruses. While traditional bacteria have been the center for biofilm study, knowledge regarding these “tiny” (CPR group bacteria and phage) residents is lagging behind. Furthermore, reproducible in vitro multispecies microbial communities are crucial to a better understanding of community-level function and dynamics. This chapter focuses on the recent findings pertaining to the biology of non-bacterial residents of the oral microbiome, such as phage and CPR group as revealed by biofilm research. It also provides an update on the newly established in vitro multispecies communities for investigating community-level function and dynamics.