This book is related to the design and application of oscillatory meso-bioreactors. in chapter one, we concentrated on the most important issues that utilizes useful oscillatory flow in meso-bioreactors. It is obvious that by analysis the geometrical design part such as free baffle area, baffle spacing, baffle thickness, oscillation frequency and amplitude. Also, the method of power input measurement in oscillatory bioreactors have introduced. In chapter two, mixing in meso-bioreactor is studied by concerning on the principles of OFM (Oscillatory Flow Mixing) and important parameters which affect the OFM and also meso-OFM, it includes; net flow Reynolds number, oscillatory Reynolds number, St, baffle free area, and other dimensional factors that should be studies before design a meso-bio-reactor. An over view of recent applications of oscillatory meso-bioreactor was shown in chapter three. Finally, in chapter four, it was concentrated on the method of microbial process simulationin meso-scale. It includes major strategies and equations such as numerical style of mass balance, biological growth and nutrient balance for bacteria.