Materials are considered with an instantaneous molecular structure describable in terms of some distribution function φ( R ,t). For such materials, a general thermodynamic theory based on the Clausius‐Duhem inequality is developed, along the lines of the thermodynamic theory for simple materials with fading memory developed by Coleman. The problem of the restrictions imposed on molecular model building by the requirement that they yield results compatible with Coleman's theory is discussed in some detail, and certain general conclusions are obtained.