Traditionally, the application of carbon fibers has been limited to very special fields such as aerospace and military because of their high cost. However, various techniques for low-cost carbon fibers are under development in terms of using low-cost precursors, low-cost manufacturing processes, and even functional coating methods. Moreover, future applications of carbon fibers can widen not only as structural reinforcements but also in information technology-based applications such as housings for electric devices, smart cloths, and healthcare items. In this chapter, we will cover the carbon fibers and their composites in recent various applications. In particular, there are classified as a low-cost production technique of carbon fibers for general industries, thin carbon fibers for extreme industries, and functional carbon fibers for smart composites.