There are 2 main airports in Sepang (45min south of Kuala Lumpur) right next to each other.  The are basically like 2 large terminal of one giant airport but are considered 2 separate airports:
KLIA (Kuala Lumpur International Airport) - the main airport for non-budget airlines in operation since 1998
and
KLIA 2 - the airport for low-budget airlines (AirAsia, Cebu Pacific, Malindo) replace the LCCT Terminal in 2014 within the KLIA complex. LCCT had been a "permanent" temporary solution for the from 2006 until 2014 and had been extremly busy being the main hub for the successfull AirAsia low-budget airline. 
 

Getting to and away from KLIA2

Train

The high-speed KLIA Express has been been extended and also connect to KLIA2. The travel time from KL Sentral (the main railway station in Kuala Lumpur) is 33min for KLIA Express and 39min for KLIA Transist and will stop at KLIA on the way. The KLIA Express can also be used to commute between the 2 airports (ticket price RM2).

• runs every 15min during peaks hours, every 20min during off-peak hours
• from Airport to City (KL Sentral) first train at 5am, last train departs 1am
• from City (KLSentral) to Airport first train departs 4.30am, last train at 1am
• detailed schedule: www.kliaekspres.com/plan-buy/schedule/
• ticket price: RM55 (children: RM25)

Bus

the ticket counters and departure bays are at Level 1

Aerobus
to / from KL Sentral (ticket price RM10, travel time approx. 1h), Genting Highlands (RM35, approx. 2h), Paradigm Mall (RM10), Premiere Hotel (RM10)
www.aerobus.my

Star Shuttle
to Puduraya (Pudu Sentral) RM10, from Puduraya RM12, Jalan Ipoh (RM10), Ipoh (RM42 / return RM75), Sitiawan (RM48 / return RM85)
transfer to and from Hotels within Bukit Bintang area (RM15, travel time approx. 1h30min)
on the way from KLIA aiport, the bus will stop outside Pudu Sentral (near Chinatown KL) and you will be transfered to a smaller shuttle bus, which will bring your to your hotel
www.starwira.com

Jet Bus
to / from TBS Bus Terminal (ticket pirce RM10, travel time aprrox. 1h15min)
www.jetbus.com.my

Buy airport bus transfer online here:

Find more bus connections here

Taxi
the taxi counters are at Level 2
Budget Taxi (max. 4 people but only 1 large piece luggage) - RM74 (Downtown Kuala Lumpur)
Premiere Taxi (max. 4 people and 2 large pieces of luggage) - RM102 (downtown Kuala Lumpur)
for larger groups and luggages there are Family Vans available
The are also metered taxi is available but use them on your own risk. For a haggle and stress-free experience get the taxi coupons from the counters at Level 1.

taxi and bus counters at Level 1

taxi capacity guideline
 

How to get from KL to KLIA Airport

How to get from KLIA to Kuala Lumpur

Gateway@KLIA2

The KLIA2 Terminal is actually only a giant shopping mall called gateway@KLIA2 disguised as a aiport or the other way around. This mall-within-airport concept spans over 3 floors and 350,000 square feet with 160 stores and restaurants and 5600 car parks spots. Knock yourself out while waiting for your flight.

gateway@KLIA2

Accommodation:

Sri Packers Hotel (nearby Backpackers Acccommodation)
Tune Hotel (large Air Asia owned budget hotel)
Capsule Hotel (container hotel on Level 1 inside the Terminal, can be booked for 3h, 6h or 12h)

there is a large outside space at Level 3 with a smoking area (in the background the tower and the main terminal of KLIA 1

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check-in counters

shops and kiosks inside the terminal