Primary production and decomposition by microbial communities underpins the functioning of terrestrial and aquatic ecosystems. Microbial communities also provide economically important services such as degradation of pollutants; direct effects on primary productivity; and indirect effects of predation, phytophagy, or resource competition. This chapter's review recent experiments with constructed communities of microbes under controlled conditions. Our review reveals that, although there are many exceptions, most studies have demonstrated a positive relationship between microbial diversity and ecosystem functioning. However, studies of natural communities have reported a variety of relationships between microbial diversity and functioning, and no consistent evidence for a significant relationship has emerged. Regarding these inconsistencies, This chapter discusses the possibility that microcosm and field studies are investigating different parts of the same underlying relationship, and also the possibility that bias in microbe culturability or error in field measurements of biodiversity make comparisons difficult.