© 2015 Elsevier Inc. All rights reserved. Environmental microorganisms normally exist as part of a community. In many cases, such communities are complex, and contain large numbers of diverse microbes that, in turn, result in natural ecosystems that provide a myriad of resources that benefit society. Fundamental to the success of these communities is their vast diversity which enables them to adapt to changing conditions. In this chapter we discuss key environmental diversity concepts including the questions of what is microbial diversity, what is a microbial community and what is a species? Subsequently, we discuss how environmental microorganisms interact with each other, with other organisms and with the environment. Community stability is also described including microbial megacommunities such as biofilms and rhizosphere communities. Finally, we discuss how diversity and interactions impact microbial adaptation, ecosystem function and the discovery of natural products.