From Brussels National Airport:

• By bus (Level 0)  Express Bus 12 to "Brussels City".
  
  This is a limited stop service which runs at regular intervals every hour until 20h00.  It stops both at SCHUMAN (roundabout flanked with the Berlaymont building and the pink marble Council of Ministers' Justus Lipsius building) and PLACE DU LUXEMBOURG. Alight at Schuman, take the Metro Lines 1 (Gare de l'Ouest) or 5 (Erasme) change at Arts-Loi (2 stops) to Lines 2 (Simonis) or 6 (Roi Baudouin) to Porte de Namur (2 stops) and take exit "Chaussée d’Ixelles". When surfacing, turn left in Square du Bastion, walk through the alleyway in front of you to Place du Champs de Mars 2 on the left. The ESTIF office is on the 5th floor.
  At other times take bus No 21 which runs every 20-30 minutes and follows practically the same itinerary, calling at every intermediate stop.
  
  

• By train (Basement Level 1).
  
  There are up to four trains an hour to the centre of Brussels (20 minute journey). Get off at Brussels Gare du Midi and take a metro lines 2 or 6 direction ELISABETH.  Get off at Porte de Namur (4 stops) and take exit "Chaussée d’Ixelles". When surfacing, turn left in Square du Bastion, walk through the alleyway in front of you to Place du Champs de Mars 2 on the left. The ESTIF office is on the 5th floor..

From Charleroi Airport (Ryanair flights):

Take the shuttle bus to Brussels Midi/Zuid Railway Station. Once there follow the white and blue M signs for the metro and take metro line 2 or 6 direction Elisabeth. Alternatively, use public transport: take a metro lines 2 or 6 direction ELISABETH.  Get off at Porte de Namur (4 stops) and take exit "Chaussée d’Ixelles". When surfacing, turn left in Square du Bastion, walk through the alleyway in front of you to Place du Champs de Mars 2 on the left. The ESTIF office is on the 5th floor.

From Gare du Midi (all international trains including Thalys and Eurostar stop there) you can:

• Take a taxi, which will take approximately 10 minutes.

• Alternatively, use public transport: take a metro lines 2 or 6 direction ELISABETH.  Get off at Porte de Namur (4 stops) and take exit "Chaussée d’Ixelles". When surfacing, turn left in Square du Bastion, walk through the alleyway in front of you to Place du Champs de Mars 2 on the left. The ESTIF office is on the 5th floor.