score:4
There are four operators of public transportation active in Brussels. However, most people only need one. But if you would have to change between operators, a ticket covering them all is available.
As complicated as this all sounds, it is a lot more easy if you are interested in the tickets. Just take a look at the fares of STIB/MIVB. Every ticket with "JUMP" in the name is valid on the entire network of STIB/MIVB (with the exception of the airport) and on the networks of SNCB/MNBS, De Lijn and TEC inside Brussels. The single tickets, return tickets and passes for a few days all have "JUMP" in their name. So you're good with any of those.
Only if you want to travel outside the borders of Brussels with either SNCB/NMBS, De Lijn or TEC will you need to buy another ticket with the relevant operator.
For completeness, I'll add a few words on the airport. The airport is not located in Brussels, but in Flanders. I've mentionned before that the airport is not covered by a normal STIB/MIVB ticket. As is often the case with airports there is a special (more expensive) fare for that. On the page with fares for STIB/MIVB that I linked to above, just look for anything with "Airport" in the name. There are trains from the airport to the city, but they also come with a surcharge and are SNCB/NMBS specific. There are also buses of De Lijn to Brussels. On those you only pay the standard fare, but they take ages and you will need to buy a ticket from De Lijn. In short, if you only visit Brussels, the airport is normally the only place where you will be confronted with a choice between the tickets of different operators.
And if you are flying to Charleroi (Brussels South Charleroi or however they may call it nowadays) we are opening another can of worms because it is so far from Brussels.
Upvote:5
Coincidentally, I was in Brussels just a few days ago and can confirm that only a single ticket is needed. You can buy them from automated kiosks in any metro station and they give you plenty of options with various number of trips and validity periods.
For a day or even two, depending on the purchase time, buying the 24h card is very cost effective. Your 24h will start on the first tapping at the entrance of metro, tram or bus and each time you tap, it will tell you how much time you have left on the card. The cost is about 7.5 Euros IIRC.
Getting out of the city or commuting between train stations, as I've seen many people do, takes another type of ticket but the metro reaches the train stations too or close by, so this is not really needed within Brussels.
Upvote:8
Update June 2023
Having recently visited Brussels again I would point out some changes which invalidate some of my answer and some of the other answers.
You can now use contactless payment. This worked for me with a non-Belgian card and the web-site claims in works with smart watches, smartphones. This is not very helpful if your bank charges a flat amount per transaction but note that STIB-MIVB bills you once per day. It caps the fare per day as well. I arrived by train so cannot independently confirm the web-site which states that contactless works from the airport.
The MOBIB basic card still exists and can be loaded with single, 10 trips and 75 trips (the five trip option appears defunct). It can also be used to load a day card and for the more expensive airport journey. What was called JUMP is now called BRUPASS. One advantage of the MOBIB card is that within the BURPASS area it can be used on buses of TEC and De Lijn and on trains of SNCB.
https://www.stib-mivb.be/article.html?_guid=d0707200-2683-3410-479e-b21a51d668f0&l=en
(That is the English page, French and Dutch are also available.)
My original 2018 answer appears below.
Public transport in Brussels is run by one organisation as far as I can tell. Certainly I have used buses, trams and the metro using the same card
https://www.stib-mivb.be/article.html?l=en&_guid=30af0085-2483-3410-5394-a71daf08acd1
If you are there for a visit you might want to get the Mobib Basic card as outlined in the link I show. You have to pay a deposit and then load it with various options: single journeys, five or ten journeys, day tickets, ... If you have the five or ten jourey option loaded you can share the card between passengers.
If you want to travel outside Brussels the position is more complicated as you need to investigate De Lijn https://www.delijn.be/en/ which covers services in Flanders.