**1. Introduction**

Specific definition of nanofibers can vary from one discipline to another, but according to one of the most common descriptions, fibers with a diameter below 100 nm are referred as nanofibers. Nanofibers have two alike dimensions (diameter) in the nanoscale and a third dimension, which is significantly larger (length). Going back to its origins, nanofibers are produced for the first time by Formhals (1934) by electrospinning of cellulose acetate solution. Although electrospinning process was used before Formhals, no one was able to form long filaments due to the use of inelastic Newtonian fluids [1]. The use of viscoelasticity in the solutions led the formation of nanofibers because the applied electric field caused a considerable reduction of the fiber diameter due to the bending instability, which is later mentioned by Reneker [2]. Forming fibers in nanoscale was a major drawback at that time, and not much attention was paid to the topic until the breakthrough of nanotechnology in the late 1990s. After almost 60–70 years later, Formhals' work was appreciated, understood, and widened [3].

Nanofibers have many advantages because of their scales, which gave them high aspect ratio (length/diameter value) above 200 and high surface area. And because almost all their properties are tunable, one can select and use nanofibers in numerous applications. The vital point of nanofiber technology is the availability of a wide range of materials such as natural and synthetic polymers, composites, metals, metal oxides, carbon-based materials, etc., which can be used for fiber production process [4].

Types of nanofibers can vary due to their nature, structure, and composition. According to its nature, one can produce natural or engineered nanofibers while one can produce nonporous, porous, hollow, core-shell nanofibers due to its structure. It is also possible to blend fiber materials to acquire a composition that can be organic, inorganic, carbon-based, or a composite [5]. When all the advantages considered (high aspect ratio, tunable properties, ability to form 3D networks, etc.), nanofibers are perfect nominee for different biomedical applications, such as tissue engineering (TE), regenerative medicine, drug delivery, nanoparticle delivery, etc. [6].

This chapter mainly focuses on the different production methods of nanofibers, their characterization techniques, recent developments in tissue engineering applications.
