Tijuana sits on the US-Mexico border, directly south of San Diego. The San Ysidro Port of Entry is the world's busiest land border crossing; expect waits of 1–3 hours heading into the US (much less going south into Mexico). Walking across from San Ysidro is often faster than driving. The Cross Border Xpress (CBX) bridge connects directly from a terminal in San Diego's Otay Mesa area to Tijuana International Airport — a remarkable bridge for people with valid tickets at either end.
From San Diego, the San Diego Trolley (Blue Line) runs to San Ysidro, then it's a short walk through the pedestrian crossing. The whole journey from downtown San Diego takes about 45 minutes.
Tijuana International Airport (TIJ) serves domestic Mexican routes and some US cities. It's also accessible via CBX from the US side.
Intercity buses from throughout Mexico arrive at the Central Camionera (main bus terminal) in the eastern part of the city.