In this chapter, vapor-grown and electrospun carbon nanofibers (CNFs) are emphasized. Fabrication processes and surface modification methods for CNFs are presented. Microstructure of CNFs is discussed based on the reported observations in various studies. CNFs have a complex structure compared to the structure of carbon nanotubes. The orientation of carbon layers in CNFs affects their mechanical properties. Experimental analyses and resulting trends are discussed from various published works, such as studies that investigate the tensile and flexural properties of individual CNFs. The range of measured properties is rather wide, which is likely due to the difference in the structure of the fibers that were tested and the presence of defects. Molecular dynamic simulation results on single nanofibers are also presented to understand their potential of reinforcing composites.