The U.S.‐Mexico Border region covers more than 2,000 miles long from the Gulf of Mexico to the Pacific Ocean; it is formally defined as the area extending 100 kilometers (62.5 miles) from either side of the international border (i.e., a 200 kilometer swath of land bisected by the international border). The U.S.‐ Mexico border has fourteen “sister cities”. Each of these sister‐city pairs form a "transfrontier metropolis, a single cross‐border functional living space with U.S. and Mexican dwellers" (Herzog 1998). San Diego‐Tijuana is the largest and wealthiest city‐region on the U.S.‐Mexico border–it has a total population of 4.1 million people (2.8 million in the San Diego metropolitan area and 1.3 million in Tijuana). The SD‐TJ city‐region's population is expected to swell to 8 million by 2030 (Kiy and Kada 2004). This AOC examines planning and development along the border including efforts to deal the following challenges: housing costs are sky rocketing; urban sprawl and a proposed triple border fence threaten rural communities and sensitive habi tats of binational ecological importance; transportation infrastructure is insufficient to service the growing traffic; and water and energy supply is becoming an increasingly urgent and hotly contested issue (Kiy and Kada 2004).