With airplane or train. Just depends from where you come and how you like to travel.
For flight information on Brussels Airport
Underground railway (metro) and buses.
Taxis are very expensive in Brussels.
We took the plane from the U.S.(the airport has a long terminal). But we found that Brussels can be a good point for daytrips to other countries. We went to Paris (train trip lasts for 1 hour 30 minutes) and Cologne (train trip lasts for approximately 2 hours) by train. Additionally, we also found out that it is cheaper to buy tickets from the Midi Station or throught the railways' websites rather than buy at the commercial websites that arranges to have the tickets sent to U.S. addresses.
The Metro and the trains. There is a ten journey METRO ticket that costs less than $10. Additionally, metro stations have a lot of art drawings in it so it is fun just riding the trams looking at the drawings. The trains are also convenient for traveling in between stations. One note: The Metro and the the train are two different modes of transportation, and have different fares. The ten-journey ticket cannot be applied to the trains travelling in between stations. YOu need to buy a train ticket that says Zone Brussels. However, in between the South Station and the North Station they rarely check tickets.
By plane: National Airport Brussels-North. From the airport you can take a shuttle-train to the centre of the city.
By train: there are three main stations in the city: Brussels-North, Centre en South. There are intercities from Paris, Antwerp, Amsterdam, London and Cologne.
By car: highways from Amsterdam, Paris, Antwerp, Cologne.
Best way to travel in the capital of Belgium is the public transport. If you have the money, you can take the taxicab, but that is more expensive than public transport like subway/underground, bus or tram. If you click on the map beside, it will be enlarged.
Most of the must-see-things can be reached by walking.
Brussels is easily reached by plane.
The major airport is Zaventem.
More info about Zaventem on my Zaventem page.
By plane, the airport Zaventem is just NE at the edge of the city (even AWACS' are landing there).
Brussels has a metro. The old center is excellent to visit on foot.