Four groups related to associators

Abstract: This is a report on associators which is based on my talk at the Mathematische Arbeitstagung in Bonn, June 2011. I first recall Drinfeld's definition of associators and explain my (and its related) results in arXiv:math/0702128 and arXiv:0808.0319. Then I explain recent developments on the four pro-unipotent algebraic groups related to associators; the motivic Galois group, the Grothendieck-Teichmuller group, the double shuffle group and the Kashiwara-Vergne group.