Hollow Fibre Membrane Contactor (HFMC) is rapidly emerging as a competitive technology for achieving mass transfer between two phases. Mass transfer takes place by passing the fluids on the opposite sides of a microporous membrane. The membrane merely acts as a physical support for the interface and does not contribute to the separation through its selectivity, as usual, the separation being primarily based on the principle of phase equilibrium. Unique features of HFMC are a well defined, constant and high interfacial area within a small volume, which can be made with higher compactness, smaller size and weight to save working space and capital investment. Moreover, flooding and foaming problems are eliminated. Other advantages over classical extractors are the possibility of operating with a wide range of flow rates, no need for density differences to achieve phase separation and no emulsification, since the interface between both phases is immobilised within the membrane pores. HFMCs are simpler and often less expensive to operate than the conventional units. They are of extreme interest because of their versatility, they can be connected in series or in parallel, and the length and diameter of the fibers and modules can be varied to provide the required contact area. HFMCs have a very broad range of applications for various types of separations. These include gas-liquid absorption, liquid-liquid extraction, pervaporation, de-gassing of liquids as well as a variety of emerging methods for treatment of fluids of industrial interest (e.g. in the biotechnological, hydrometallurgical, petrochemical fields) and of biological significance.