(/ˈaɪnstaɪn/ EYEN-styne;[4] German: [ˈalbɛʁt ˈʔaɪnʃtaɪn] (listen); 14 March 1879 – 18 April 1955) He was a German-born theoretical physicist widely acknowledged as one of the greatest and most influential physicists ever. Einstein is best known for developing the theory of relativity, but he also contributed significantly to developing the theory of quantum mechanics.