Publisher Summary This chapter discusses process technology of biotechniques. The number of applications of biological treatment systems for off-gas purification has strongly increased. The application of a biological treatment system is based upon the microbial degradability of the compounds present in the waste gas. The performance of a continuously operating bioreactor is the ultimate result of a complex interaction between the microbiological and physical phenomena, called as the “macrokinetics” of the process. The microkinetics of the degradation process are generally investigated and modeled for pure cultures of suspended microorganism. In many bioreactor systems the degradation also takes place within fixed biofilms, which means that additional mass transfer phenomena (i.e., mass transfer to the biofilm and internal diffusion) should be taken into account. The elimination of organic substrates by microorganism results from the fact that those organisms can use the organic compounds as its sole energy—catabolism, and carbon source—anabolism.