SURFace ACTive AgeNTS, surfactants, are a larger class of molecules which have significant technological and biological importance. They are characterized by their dual properties, one part consisting of a hydrophilic (water loving) and a hydrophobic (oil/air loving) part. This amphiphilic nature of surfactants is responsible for their association behavior in solution (micelles, bilayers, vesicles, etc.) and their accumulation at interfaces (gas/liquid, liquid/liquid or liquid/solid). The association behavior of surfactants in solution and their speed and affinity for interfaces is determined by the physical and chemical properties of the hydrophobic and hydrophilic groups respectively. The driving force behind the association is the reduction of the free energy of the system. Therefore, when a surfactant comes in contact with water it accumulates at the air/water or oil/water interface causing a decrease in the interfacial tension. The BPA-800P from Attension characterizes surfactants by determining the dynamic surface tension. The semi-dynamic, static surface and interfacial tensions as well as critical micelle concentrations can be measured by using the Theta and Sigma series tensiometers.