Nanofillers, particularly those composed of carbon nanotubes and clays, have received a lot of attention recently. To be considered nanoparticles, a substance must have at least one dimension between 1 and 100 nm and all other dimensions must be more than 100 nm. A further division into nanoparticles, nanofibers, and nanoplates is possible depending on the number of dimensions present. Despite all the recent media attention, it is important to keep in mind that nanofillers are nothing new and that they already have a significant commercial presence. These include carbon blacks, silica that has been both precipitated and fumed, and calcium carbonates. Employing synthetic polymer nanoparticles has garnered renewed interest recently. A number of cutting-edge composite applications hold promise for nanoplates like graphite and eventually graphene. Most likely, limitations will result from cost (and coloring). The same problems will affect carbon nanotubes and fibers, which are analogues of graphene fiber. The extraction of nanocrystals from abundant plant sources, such as starch and cellulose, is another interesting issue, especially from the perspective of sustainability. Despite some promise, current production techniques are costly and far from being entirely “green.” They have trouble redispersing into polymer matrices, much like nano clays do.