The well-established physicochemical techniques presently available for treatment of H2S-containing gases are many and varied, reflecting the significant variations in the nature of the gas streams to be treated. The continuing demand for improving process economy has led to investigations into microbiological alternatives, and a variety of biological gas desulphurization processes have been proposed. Phototrophic as well as heterotrophic and autotropic microorganisms have been utilized and, based on the inherent differences in nutritional requirements and abilities to catalyze specific reactions, different process schemes have been established and investigated. Only very few of the biological processes have been proven an a large scale, and much optimization work still remains to be done before one of the biological alternatives may be chosen as the optimal one for a certain application.