Microbes exist in complex, multi-species communities with diverse interactions that play an essential role in both human health as well as the health of the planet. Over the last decade tremendous progress has been made in characterizing these communities, but the lack of experimentally tractable model systems has made it difficult to discern the rules governing microbial community assembly and function. In this talk I will describe our recent experimental efforts to develop a bottom-up approach to understanding the dynamics of these communities. We have begun by quantifying the network of pairwise competitive outcomes among species within a model microbial community. We find that simple assembly rules incorporating just these pairwise competitive outcomes are surprisingly successful in predicting the outcome of multi-species competition, indicating that higher-order interactions among species can often be neglected. While deterministic dynamics are sufficient to explain these experiments, we find that stochastic effects can dominate during colonization of a new environment such as the gut of the worm C. elegans, leading to strong heterogeneity between the microbial communities in individual animals. These results illustrate how a bottom-up approach of characterizing individual interactions can explain the emergent behavior within complex multi-species communities.