This book describes the microemulsion phenomenon in a systematic manner and not only provides an up-to-date introduction to this topic but aslo serves as the basis for further development in the area.The progress of microemulsion research has taken place in well-defined stages. The introduction period was founded on Schulman's original discovery and was, as expected, focused on the interfacial free energy. Because Schulman obtained his microemulsions from a macroemulsion by the addition of a cosurfactant.The present stage is characterized by an extensively enhanced knowledge about structure and dynamics in these systems. This has led to the realization that the structure of the microemulsions is related both to solutions with critical behaviour and long range order structures, the lyotropic liquid crystals. These two aspects have been elucidated independently by the French groups and by the Lund Spectroscopy group.