No biological system involves a single cell functioning in isolation. Almost all consist of highly connected networks of interacting individuals, which respond and adapt differently to signals and conditions within their local microenvironment. For example, human tissues and their cancers contain mosaics of genetic clones, and the transcriptomic and metabolic profiles from genetically identical cells are also highly heterogeneous. As the full extent of multi-cellular heterogeneity is revealed by recent experimental advances, computational and mathematical modelling can begin to provide a quantitative framework for understanding its biological implications. In this talk, I will describe some functional aspects of multi-cellular heterogeneity and explore the consequences for human health and disease.