LEDs are used in communication systems transmitting low and medium data rates (< 1 Gbit/s) over short and medium distances (< 10 km). These communication systems are based on either guided light waves (Keiser, 1999; Neyer et al., 1999; Hecht, 2001; Mynbaev and Scheiner, 2001; Kibler et al., 2004) or free-space waves (Carruthers, 2002; Heatley et al., 1998; Kahn and Barry, 2001). In guided-wave communication, individual optical fibers or fiber bundles are used as the transmission medium and LED-based optical communication links are limited to distances of a few kilometers. Optical fiber systems include silica and plastic optical fibers. Free-space communication is usually limited to a room, even though longer distances are possible. In this chapter we discuss the characteristics of transmission media used for LED communication.