Solid state 2H NMR observation of deuterium labeled analogs of lipids provides a superb, non-invasive probe of molecular organization in membranes. This talk will illustrate the approach with results on what omega-3 polyunsaturated fatty acids (n-3 PUFA), the bioactive ingredient in fish oils, do to membranes following their uptake into phospholipids. Our experiments on lipid bilayers reveal an underlying role in regulating the size, and potentially the functionality, of membrane domains. Central to this behavior is the high disorder possessed by PUFA and the aversion for cholesterol that it creates.