This chapter overviews a history of optical fibers by highlighting typical optical fibers and introduces some cutting-edge plastic optical fibers (POFs). Section 25.1 briefly remarks recent trends of optical communications. Section 25.2 explains basic structures of optical fibers and their characteristics. Two most important characteristics, transmission loss and capacity, are introduced in Sects. 25.3 and 25.4 by focusing on POFs. Section 25.3 divides the transmission loss into intrinsic and extrinsic factors. It mentions further classification into absorption and scattering losses, followed by low-loss POFs. Section 25.4 describes transmission capacity in terms of modal and material dispersions and then discusses high-bandwidth POFs. Section 25.5 introduces a novel GI POF that positively utilizes light scattering, conventionally considered a negative factor. Its large light scattering induces strong mode coupling, reducing interference noises. The GI POF has successfully achieved an error-free PAM-4 data transmission without FEC, realizing high speed, small delay, and low power consumption systems for the first time.