Born Mörby, Sweden, 1894, as the youngest son of Sweden's first rabbi, Gottlieb Klein. From the age of 15 he worked, on Svante Arrhenius' invitation, at the Nobel Inst. on the solubility of salts with radioactive indicators. From 1918 Klein frequently visited Copenhagen and he stayed there after completing his doctoral dissertation at Stockholms Högskola, 1921. His interest turned to quantum theory and with Svein Rosseland, he introduced "collisions of Lund U., 1926. Introduced relativistic wave edquation (Klein-Gordon equation). Lecturer NBI 1927. Deeply involved in Bohr's work on correspondence and complementarity. The Klein-Nishina formula (1929) convinced many physicists of the soundness of Dirac's relativistic equation, in spite of difficulties, one of them know as Klein's paradox. Prof. math. phys. Stockholm Högskola 1930-1962. Worked on statistical mechanics, superconductivity (with Jens Lindhard, 1945), distrubitions of chemical elements and cosmology (cosmological model with Alfvén, 1963). Died Stockholm 1977.