Nanotechnology has become an increasingly popular topic in the past couple decades; extensive literature has been published detailing ‘nanocomposites’. In nanocomposites, the term nanofillers is often described as a material in the nanometer scale made up of discontinuous phases, which is embedded into a continuous phase (matrix), improving the properties of the matrix. The use of nanofillers has commonly shown to improve mechanical properties, thermal resistance, barrier properties, and wear resistance. The region the nanofiller material and matrix interact occurs at a nanoscale. The interface between the two phases is considered the ‘interphase’ and is an important factor on the final properties of the nanocomposite. Nanofillers are commonly added to resins or binders such as polymers, concrete, and composites. The use of nanofillers can improve specific properties or reduce cost of material and in certain instances both. This chapter provides insight into the fabulous world of nanofillers. It provides a theoretical outlook on aspects relating to nanofillers in laboratories, their synthesis, and applications in fabrication. It also outlines fundamentals on the interphase. The various forms of nanofillers are explained extensively in the chapter.