The outer membrane of Escherichia coli protects the cell against hostile agents and facilitates the uptake of nutrients. This activity is mediated by macromolecular structures called porins. Porins are not very selective and have only some specificity towards cations or anions. The porins represent ideal systems for addressing the fundamental electrostatics principles governing ion flow in molecular pores. Because high resolution well-characterized structures are available for outer membrane porins of E coli, they provide excellent model channel systems for computations based on detailed atomic models. We are currently working on the cation-selective matrixporin (OmpF) which is a major component of the E. coli outer membrane.