- nearly every bit of data (including phone calls) traveling around the world flows through - you take a glass rod, heat it, and stretch it - making a thin strand of glass many miles long - you stick a laser diode at one end, and light sensor in other end - flashing the laser means 1, off means 0 (sends digital data) - compared to [[Copper]]: - lighter, less expensive, immune to electromagnetic interference - carries far more data on a single line