The biological production of renewable fuels and chemicals, medicines, and proteins is not possible without a properly functioning bioreactor. Bioreactors are expected to meet several basic requirements and create conditions favorable to the biological material such that the desired production is maximized. The basic requirements, which are strongly influenced by fluid mechanic principles, may include minimum damage to the biological material, maximum reactor volume utilization, optimized gas-liquid mass transfer, and/or enhanced mass transfer from the liquid to the biological species. Each of these goals may be achieved within any of the major bioreactor designs, which generally fall under the categories of stirred tank, bubble column, or airlift bioreactor. Yet, each of the bioreactor designs has strengths and weaknesses. This paper provides an overview of bioreactor hydrodynamic developments and the fluid mechanic issues that should to be considered when selecting a bioreactor for experimental and production purposes.