Large multi-species communities of microbes are exploited in a wide range of long-established and newly emerging technologies, such as wastewater treatment, bioremediation and electricity generation from wastewater using microbial fuel cells. However, the coming together, or assembly, of large communities in natural environments and the fluctuations in their composition and structure, once assembled, is still poorly understood. The remarkable success of some of the more established technologies, such as wastewater treatment, can largely be attributed to the wealth of empirical knowledge that has been built up through many years of research and application. However, such a climate of empirically based technological evolution, makes difficult the translation of any insights from similar strategies in different environments. Furthermore, industry's adoption of new technologies is slowed by our inability to quantify both the common and key processes in microbial community assembly and the relationships between community composition, structure and function. Therefore, satisfactory mathematical descriptions of the microbial world are now the bottleneck in successfully engineering microbiological systems. A theoretical basis for predicting and manipulating microbial community composition and structure will have a wide application: setting established practices on a new theoretical foundation; quantifying the risk in emerging environmental biotechnologies; and potentially opening up whole new technologies. The research proposed in this Advanced Fellowship application aims to build upon previous successes in modelling microbial community assembly and develop further quantitative, testable theories for the microbiology of biological waste treatment systems.