Book excerpt: The Boundary Flux Handbook summarizes over 90 chemical compounds (single compounds, in mixed solution) and different feed streams from industry (complex streams exiting the agricultural, biological, pharmaceutical, food, dye, textile, etc. industries) and the respective critical and threshold fluxes (hereafter referred to the boundary flux), together with data on rejection and permeabilities. It gives the reader valuable data to quickly estimate membrane performances in respect to productivity, selectivity and longevity, and permits to estimate capital and operating costs of membrane processes during the plant design operations. The book is valuable for scientific researchers and chemical engineers working with membrane technology as a reference for lab work, membrane process design and control. For each specific feedstock, only the best performing membrane(s) are listed. At least one commercial membrane module and a maximum of 4 different choices were reported, in order to assure the possibility to easily integrate the proposed membranes into the systems by contacting the relevant commercial partner, and to define the best choice of the pretreatment steps, permitting quick membrane process optimization or revamping operations.