Filtration efficiency depends greatly upon selecting the proper media, a decision that requires the careful consideration of the advantages/disadvantages of each. The four major criteria for evaluating filtration media are: permeability, filtration efficiency, filtration capacity (or life) and filtration cost. In this article, Dr. Ernest Mayer and Jennifer Warren of E.I du Pont de Nemours and Company, USA compare the major characteristics of the two most commonly used filtration media groups: microporous/PTFE (polytetrafluoroethylene) membranes and nonwoven materials; and discuss ways of evaluating them. A new nonwoven material, Tyvek(R) SoloFlo(TM), formulated by DuPont for use specifically in the filtration industry is also introduced.