Overmining of cultivable microorganisms was largely responsible for the end of the golden age of antibiotic discovery. Uncultivable species represent ~99% of the total microbial diversity and are a potential source for novel pharmaceuticals, including antibiotics. Methods to grow “uncultivable” bacteria in situ have been developed. Previously uncultivable organisms form colonies in a diffusion chamber placed in their natural environment, such as marine sediment or soil. Filamentous actinomycetes or fungi will penetrate into an empty diffusion chamber placed in soil and form colonies as well. Growth in co-culture results in cultivation of isolates on synthetic media as well. In the marine sediment environment, growth promoting factors from helper organisms appear to be siderophores. Apparently, uncultivable species lost their ability to make siderophores and became strictly dependent upon their neighbors. The majority of microorganisms tested were also able to adapt to growth on synthetic media upon repeated chamber-to-chamber reinoculations. The chamber apparently acts as an intermediate between the natural environment and a Petri dish, selecting for variants that are able to grow in vitro. In collaboration with NovoBiotic Pharmaceuticals, we have been using these approaches to screen uncultivable microorganisms for antimicrobial activity. The hit rate from this source is comparable to the hit rate from cultivable species.