The basic issues involved in designing membrane-based extraction processes are systematically treated. The main objectives of liquid-liquid extraction operations (i.e. maximization of solute recovery and of solvent saturation by solute) serve as the basis of this presentation. Analyzing the operating characteristics of a single membrane module (comprised of a bundle of parallel fibers), it is shown that two parameters determine process performance; i.e. the extraction factor E (reflecting thermodynamic limitations) and a lumped parameter A which takes into account geometrical features of the module as well as kinetic and thermodynamic limitations. A narrow range of parameter values for E and A, leading to near-optimum module performance, as well as a procedure for preliminary design are recommended. The relative significance and inter-relation of geometric and operating variables is also appraised. Using dimensionless quantities, closed form expressions of general validity are developed, for a variety of continuous process options (single module, multi-module, counter-current and "cross-flow" arrays). These expressions allow direct comparative assessment of design alternatives, thus facilitating process design and performance optimization. Batch operations are also treated for the two practically significant cases of solvent re-circulation and "once-through" mode of operation. Limitations of the former and merits of the latter mode are identified. (c) 2005 Elsevier B.V. All rights reserved.