The airport is 20km from Mumbai city center.
Taxi - this is probably the best option to reach downtown Mumbai. You have 2 choices: prepaid taxis, whose counters are in the Arrivals area of each terminal; and metered taxis, which can be hailed outside the terminal buildings. Auto rickshaws are also available outside both terminals.
Bus - local BEST buses connect both terminals with Mumbai destinations. MSRTC and private operators provide longer distance services.
Train - the nearest stations (Vile Parle East and Andheri East) can be reached from the airport by bus, taxi or auto rickshaw.
Car rental - there's just 1 car rental service available at Terminal 1 Arrivals, while Terminal 2 has a choice of 2.