Cancun International Airport (CUN) is the second-busiest in Mexico and is served by direct flights from across the US, Canada, the UK, and continental Europe. It sits 20 km south of the Hotel Zone; ADO operates the public bus to downtown and the Hotel Zone (roughly half-hourly, around 90 pesos), official taxis and shuttle vans handle the rest. Be wary of timeshare touts in the arrivals hall — they will offer you a free transfer in exchange for a sales presentation.
Long-distance ADO buses connect Cancun with Mérida (4 hours), Playa del Carmen (1 hour), and Tulum (2 hours) from the downtown bus terminal. A drive from the US border (Brownsville or Laredo) takes three full days.