San Diego (pronounced /ˌsæn diːˈeɪɡoʊ/), named after Saint Didacus (Spanish: Diego de Alcalá), is the eighth largest city in the United States and second-largest city in California, after Los Angeles, with a population of 1,359,132 (Jan 2010) within its administrative limits on a land area of 372.1 square miles (963.7 km2). The urban area of San Diego extends beyond the administrative city limits with a population of 2,880,000. Also, this is part of megalopolis (San Diego and Los Angeles metropolitan areas) with a population of about 22 million. Located along the Pacific Ocean on the west coast of the United States.

Tijuana (English pronunciation: /tiːəˈwɑːnə or tiːˈwɑːnə/, Spanish: [tiˈxwana]), is a city on the Mexican Pacific Coast located in Baja California. The city is the largest city of the Mexican state of Baja California. On a larger scale Tijuana is the 28th largest city in the Americas and is situated on the United States–Mexico border adjacent to its sister city of San Diego, California. Tijuana is the westernmost city in Mexico, though the westernmost population center is located in Isla Guadalupe, and is one of dominant regional centers in northwestern Mexico. The city is currently one of the fastest growing cities in Mexico.

According to the 2005 census, the Tijuana metropolitan area was the sixth-largest in Mexico, with a population of 1,483,992. 2009 National Population Council (CONAPO) place the Tijuana metropolitan area at 5th largest in the country with 1,784,034 persons. The metropolitan area is comprised of two municipalities: Tijuana and Playas de Rosarito.

Tijuana also forms part of San Diego-Tijuana, a metropolitan area with the total population estimated to be just over five million in 2009 and approximately 5,105,769 in 2010, making it the second largest metropolitan area in the Californias, 22nd largest metropolitan area in the Americas and the largest bi-national conurbation that is shared between US and Mexico. It is estimated that the two border crossing stations in Tijuana account for 300,000 daily border crossings from Greater San Diego.