Compared with thermoplastics (TP), the morphology of thermosets (TS) has not been studied thoroughly. Most TS are amorphous and composed of highly crosslinked molecular networks. But the concept of homogeneous inﬁnite networks represented by one giant molecule (Chapter 3), has long been questioned. For example, epoxy networks (the most studied networks) are very often reported in the literature as inhomogeneous. Historically, this claim was supported by electron microscopic observations of free and fracture surfaces of epoxy networks, revealing the presence of a nodular morphology in the range of 10-100 nm. Nodules were suggested to be sites of higher crosslink density, resulting from intramolecular crosslinking and cyclization reactions. This conclusion was supported by the observation that, upon etching, the internodular material was preferentially attacked (Racich and Koutsky, 1976).