Central to understanding the behavior of microbial communities (MC) are microbial interactions and their organization. In my talk I will give two examples where community-wide interactions lead to complex behavior of the MC. Pathogenic microbiota cause chronic infections in the airways of cystic fibrosis patients. We show that organization into two competing communities and shifts in their interactions, as well as in their metabolic core processes are associated to a shift in the severity of the disease. Using network analysis and the keystone concept, we propose functional and taxonomic keystones as targets for novel drug development. The human gut microbiome is characterized by seemingly opposing trends: stability of a (functional) core community and pronounced variability throughout lifetime and between subjects. We explore self-organization as potential mechanism underlying these evidences. We use a modeling approach to analyze the time behavior of gut microbiota and find that the community stratifies into three sub-groups linked to lifetime and abundance. In time the gut community exhibits pink noise and scale-invariance, hallmarks for self-organization.