Introduction: The interest in studying surfactant aggregation in other polar solvents than water has increased quite recently; indeed, much of the advances in the field are less than ten years old and the majority of that has been published during the last years. The research can be motivated, mainly, with two different types of arguments. First, the basic consideration of amphiphile aggregation such as a description of the hydrophobic interaction leading to e.g. micelle and liquid crystal formation. What can be learned from comparing water with other polar solvents? Much work has been performed to elucidate what properties of the solvent that are essential in order to obtain a hydrophobic (or "solvophobic") interaction. Comparisons of critical micelle concentrations in different solvents with parameters characterizing the solvent are numerous in the literature Second, there are technical applications where amphiphile aggregates and structures are needed to promote a specific effect, while circumstances may prevent the particular use of water due to certain reactions, corrosion problems and so on. Of particular interest in this context is e.g. alcohol based systems for cleaning purposes; another interesting area is chemical reactions in an aprotic solvent such as formamide. This review will deal with the first, fundamental point, in particular the formation of micelles and the mapping of phase equilibria. This is a logical starting point, since a prerequisite for most applied work in the field is some knowledge of the relevant phase diagrams. Methodological questions have often been raised when studying non-aqueous systems, since many early studies on micellization were performed using indirect methods for detecting aggregation. This has caused considerable confusion due to apparently unreconcilable results. Also, recently, several studies have pointed out the difference between a proper micellization process and ordinary aggregation. As will be discussed later, depending on combination of solvent and surfactant, it is possible to have a cooperative aggregation (micelle formation) as well as a more gradual aggregation process.