Absorption is one of the unit operations which involve transfer of one (or) more components of a gas stream to liquid stream. Stripping is the opposite phenomena to absorption; in this operation one or more components of a liquid stream are removed by vaporization into an insoluble gas stream. Both absorption and stripping can be operated as equilibrium stage operations with constant rate of liquid and vapour streams. The performance of an absorbers / strippers may be evaluated using mass transfer coefficients (MTC). In addition to MTC, xy (or) XY plots are used for the performance evaluation. The performance may be evaluated by graphical or analytical methods. The graphical techniques are developed by Lewis whereas analytical method proposed by Kremser, which uses absorption factor in their calculations.In this book both approaches are covered with basic details and examples. This book is intended for students, professionals and academicians in the Chemical Engineering,Mechanical Engineering, Biochemical Engineering, Biotechnology, Food Engineering, Petrochemical Engineering, Polymeric and Pharmaceutical Engineering, Streams.