The Slav defence is an important reply to 1d4. It is popular with top players. The Slav usually begins:

1.d4 d5
2.c4 c6 (this is the characteristic move of the Slav)
3.Nf3 Nf6
4.Nc3 (see diagram)

Other fourth moves for White are 4.e3, 4.cxd5 and 4.Qb3.