The theory of continuous distributions of inhomogeneities in solids can be traced back to the work of Kondo, Bilby, Kroener and other researchers in Japan and Europe during the 1950's and early 1960's. In a seminal paper published in 1967, Noll presented a somewhat different perspective. In recent years, the theory has been expanded to include materials with internal structure and, to a certain extent, functionally graded materials. Moreover, the inclusion of the concept of material evolution has permitted to motivate the natural emergence of the concept of Eshelby stress as the driving force behind the motion of defects. These and other aspects will be presented and discussed.