We talk about a convergence of kinetic vorticity of Boltzmann toward the vorticity of incompressible Euler in 2D. When the Euler vorticity is below Yudovich, we prove a weak convergence toward Lagrangian solutions, while for the Yudovich class we have a strong convergence toward a unique solution with a rate. The talk would be self-contained covering necessary background in basic Boltzmann theory, asymptotic expansion (Hilbert expansion), and Lagrangian solutions of Euler.