Flights and trains
Unless you choose an adventurous way of entering the country via one of the few border points open with Russia or China you basically have just two choices to enter the country:
Either by flying to Mongolia´s only international Airport in Ulaanbaatar or by taking the Mongolian branch of the Transsiberian Railway either from Russia of from China.
A few tips and general oberservations:
The Ulaanbaatar - Beijng leg of the Transsiberian Railway tends to be booked out in the summer. Try to book as early as possible.
Here a link to a useful website: Transsib Timetable
As at the time of this writing (September 2013) only two airlines had direct flights from Europe to Mongolia:
These were Aeroflot with a stopover in Moscow and MIAT the Mongolian national carrier that likewise stops over in Moscow. Miat is generally more convenient to fly with but doesn´t have the extensive network of the Russian carrier.
From the rest of the world there are direct flights by Air China , Korean Air and United Airlines.
In the height of summer flights to Mongolia and especially MIAT are often booked out. Therefore we strongly recommend to book as early as possible.