The evolution of vehicle electronics has resulted in corresponding changes in how vehicle communications networks are being used. Initially used for diagnostic and low speed data sharing applications, networks are increasingly being used for real time control applications that require significantly higher data rates. While high speed networking technology has been primarily developed for office LAN and telecommunications applications, the requirements of in-vehicle communication networks differ significantly from their office and factory counterparts. This paper will examine the issues involved in choosing a physical layer for a vehicle network including: media bandwidth, electromagnetic compatibility (EMC) requirements, fault tolerance, serviceability requirements, ability to meet vehicle environmental requirements, network architecture limitations, and cost. This paper will also show that for application data rates above 250Kbps, plastic optical fiber (POF) is the lowest cost, highest performance network medium. It will also show that, unshielded twisted pair wiring (UTP) provides the best solution for data rates under 125KBps. For data rates between 125Kbps and 250Kbps, the optimal medium is application dependent.