This lecture covers the importance of gas transfer in bioreactors, including oxygen transfer to biological processes, stripping of volatile toxic organics, CO2 exchange for pH control, ammonia and odor removal, and chlorination for disinfection. It explains the mechanisms of gas transfer, dissolved oxygen uptake rate, phase equilibrium assumptions, and empirical correlations for estimating oxygen transfer coefficients. The lecture also discusses reaction kinetics that incorporate aeration based on substrate consumption, using a double Monod type relation.