Bioreactor configuration is closely related to the effective gas-liquid mass transfer. Thus, reactor design plays an important role in syngas fermentation. High mass transfer rates, high cell densities, low operation and maintenance costs, and easy scale-up are some of the key parameters for designing an efficient bioreactor system. Similarly, the bioreactor size greatly depends on the rate of mass transfer for sparingly soluble gases (Klasson et al., 1992; Vega et al., 1990). Some of the commonly used reactor configurations are discussed in the following section.