CP Violation in the $B$ system with the LHCb experiment

During 2011, LHCb has collected an integrated luminosity of 1.1 fb$^{-1}$, giving rise to a large variety of measurements. Amongst these, measurements of $CP$ violation in $B$ decays play a central role. Three highlights are presented in this paper. An evidence for $b \to u$ transitions in $B^{\pm} \to DK^{\pm}$ is obtained, confirming recent BELLE results. A first evidence for direct CP violation in the $B_s^0$ system is presented and CP violation is observed in the $B^0$ system, using respectively $B_s^0 \to K \pi$ and $B^0 \to K \pi$ decays. An unambiguous world leading measurement of the mixing phase $\phi_s$ of the $B_s^0$ system is obtained by looking at $b \to c\bar{c}s$ transitions.