Airport Bus
The key airport ground transport option
The STM Express Bus 747 (24hrs, every 10-15mins; $11 one-way) provides convenient and inexpensive access in and out of the airport to points Downtown and the Central Bus Station in the Latin Quarter (505 de Maisonneuve Blvd. E. / Berri-UQAM Metro station).
In peak traffic times you may find it best to get off at Lionel Groulx subway station and use the Metro from there.
Day ticket
Your airport bus ticket serves as a day ticket for the entire Montreal subway and bus system, so if you have one make the most of it in the 24hrs after you arrive. If you're buying one to leave – then buy it up to 24hrs before you leave to make the most if it then too.