Fiber-optic networks are the backbone of Internet and telephone communication lines. Total internal reflection guides light in silica fibers. Low absorption makes transmission over 100 km without amplification possible in single-mode fibers. Distributed-feedback lasers with modulators put up to 10 and soon probably 40 Gbit/s onto a single wavelength. The signal is guided through the fiber, maybe repeated and amplified many times, until it reaches its destination.