It is well known that increasing the ionic strength of aqueous solutions containing certain categories of anionic surfactant can produce interesting behaviors. The molecules of some surfactants cluster together forming spherical micelles. However, a select few surfactants, with particular molecular structure, undergo a remarkable transition from spherical micelles to larger, anisometric aggregates. The size, flexibility and extent of interaction of these aggregates all have an influence on the rheological properties of such solutions, producing very substantial viscosities at low shear rates. Conversely, when these surfactant solutions encounter other chemical species, particularly relatively non-polar materials, like alcohols, glycols and hydrocarbons, this affects the shape and structure of the micelles. As a result, the phase behavior is altered and the solution undergoes a dramatic reduction in viscosity. Proper selection of a surfactant allows its application in several oil field treatments such as reservoir gravel packing, frac-packing, fracturing, brine thickening, non-damaging temporary plugs and also for reservoir flooding and water shut-off. The main advantage of these solutions, compared to conventional polymer systems, is the potential for reduced formation and proppant pack damage. However, there are many other advantages. These fluids exhibit unexpectedly low high-shear viscosities resulting in low friction pressures, even in small tubular. In addition, due to the very low viscosity of the broken fluid, faster load recovery of injected fluids is possible. A final benefit offered by these systems is operational simplicity at the well site, since there is no need to "pre-gel" tanks ahead of the treatment. This paper describes the chemistry involved to develop these viscous solutions and their applications in different treatments in the field.