Gas diffusion/permeation techniques involving different membrane interphases or an open fluid/fluid interface are presented as powerful tools for separation and/or preconcentration of a variety of classes of analytes, i.e., gases, gas-evolving species, volatile inorganic and organic substances, etc., from gaseous, aquatic, or organic sample matrices. Principles of the techniques and factors influencing transmembrane transport, such as acidity of the donor and the acceptor streams, their flow rates, membrane characteristics, temperature, pressure, etc., are described. Different configurations of FIA manifolds for stopped-flow, closed-loop, and continuous flow arrangements, designs of principal instrumental modules, characteristics of different semipermeable membranes, and special arrangements of detection systems are outlined.