The recovery of various metals from primary and secondary resources becomes increasingly important, especially for low concentrated resources, where the hydrometallurgical recovery route is economically preferable over the pyrometallurgical. Solvent extraction is a crucial process step in concentrating and purifying metal ions from dilute or multi-component feed streams. For an industrial design of extraction processes and equipment several basic data are necessary. The methodology of apparatus design, the scale-up challenge, and the toolbox of available methods are discussed exemplarily for the solvent extraction of tungsten.