Publication Order

The Lion, the Witch and the Wardrobe (1950)
Prince Caspian (1951)
The Voyage of the Dawn Treader (1952)
The Silver Chair (1953)
The Horse and His Boy (1954)
The Magician's Nephew (1955)
The Last Battle (1956)

Chronological Order. Later printings are often numbered by the timeline of the Narnian world itself. If you prefer to read the history of Narnia from its creation forward, use this sequence:

The Magician's Nephew
The Lion, the Witch and the Wardrobe
The Horse and His Boy
Prince Caspian
The Voyage of the Dawn Treader
The Silver Chair
The Last Battle (1956)