The benefit of achieving multifuctionality has been a strong driving force behind increasing interest in lightweight polymer matrix nanocomposites. Versatile, lightweight structural designs can be developed through tailoring the mechanical, thermal, and electrical properties of polymer matrix composites, especially those utilized in the aerospace, marine, and automobile industries. Utilization of novel reinforcement materials such as carbon-based nanostructures in an effort to enhance a composite’s properties has proven to be useful for a plethora of structures with strict design requirements. This chapter intends to provide an opening review on polymer matrix composites, especially those reinforced with carbon microfibers and carbon nanofibers (CNFs). Some of the recent applications of carbon microfiber reinforced composites in modern automobiles are shown. However, the desire to fabricate composites with further improved properties and performance has led to exploration of CNFs as a new reinforcement. Cost and weight-saving remain crucial factors that determine industrial viability of CNFs. Through the use of reinforcement such as CNFs, resultant composites have been shown to exhibit enhanced properties such as strength, thermal stability, and thermal and electrical conductivities, all with negligible variation in the density.