The best way to get around Lille is by riding the metro. The metro is cheap, easy, and fast. It is a much smaller and less complex system than that of Paris, London, or New York for example. There are only two lines. To ride the metro you must buy and compost a ticket before you board. I would suggest buying day passes for the metro if you'll be hopping on and off frequently. These cost much less than if you had to buy a seperate ticket each time. You can buy these on any of the automated machines in the metro stations.
The bus system is also well used and organized in Lille, but I found it more confusing than the Metro. And of course there is always a lot of walking involved, my favorite form of transportation. :)
The photo shows a few of the main attractions in Lille and the map of the centre ville.
