The principles and uses of membrane filtration in the preparation of pharmaceutical solutions are discussed. The preparation of pharmaceutical solutions often requires their passage through a thin polymeric membrane containing many tiny pores. The purpose is to remove viable and nonviable particles in order to clarify or sterilize the solution. Particles may be retained by sieving, entrapment, or electrostatic attraction. The largest pore size that will yield a sterile filtrate is 0.2 microns. Membrane filters are either hydrophobic or hydrophilic. The rate of flow through a filter is affected by the resistance of the filter, the viscosity of the solution, and pressure. Filters are commonly composed of mixed esters of cellulose, polysulfone, polyvinylidene difluoride, nylon 66, polycarbonate, or polytetrafluoroethylene. In selecting a membrane filter, a pharmacist must consider pore size, compatibility, fluid volume, particulate load, and the filter holder. Various tests are available to evaluate the integrity of filters. Filters are useful in testing end products for sterility. The use of an in-line filter during the administration of large-volume injectable solutions can prevent the introduction of particles, air, and microorganisms into the patient. Therapies in which a 0.2-micron filter may be contraindicated include lipid emulsions, low-dose infusions, low-volume infusions, drugs for which the pharmacologic properties are altered by the membrane filter, and drugs that adhere to the membrane. Pharmacists need an in-depth understanding of the principles of filtration, the characteristics of filters, and their use in the filtration of pharmaceutical solutions.