Publisher Summary This chapter discusses the different types of continuous culture in ecological studies. Chemostat has become the most widely used apparatus for studying microorganisms under constant environmental conditions. The level of complexity of most natural ecosystems is such that relatively few microbial ecologists are courageous enough to endeavour direct experimenting in the field. They choose to study microbial ecology in the laboratory. The approach may seem illogical, but has resulted in the gathering of an enormous amount of information on how microorganisms thrive in their natural habitats. It is certainly true that the use of continuous cultures has contributed immensely to the success of this approach. It is indeed through the principle of continuous cultivation under precisely defined conditions that particular environmental influences can be singled out and studied in detail in the chapter. Thus, the use of chemostats demonstrates that in spite of its “utterly unnatural constancy and homogeneity,” it does make it possible to obtain some understanding of microbial life in nature. Yet, this is not the reason for microbial ecologists to revel in complacency.