We will recall Cohomological Hall algebras (COHAs) introduced by Kontsevich and Soibelman and describe the structure of COHA of a symmetric quiver, proving Kontsevich-Soibelman conjecture. This is a recent preprint arXiv:1103.2736. We will also explain how this implies Kac's conjecture for quivers with enough loops, as it was shown by S. Mozgovoy.