DNA sequencing has revealed stunning diversity in many microbial biofilm systems from wastewater treatment and paper mills to dental plaque and shower curtains. What are the physical, chemical, and biological phenomena that allow coexistence of such a variety of microbial species in microscale proximity? Some answers to this question include: concentration gradients of metabolites within the biofilm deriving from reaction-diffusion interactions, lateral heterogeneity of substratum colonization, attachment and detachment phenomena, cell mobility inside the biofilm, and dormancy. These mechanisms are explored at a conceptual level and in numerical simulations.