Christian Felix Klein (25 April 1849 - 22 June 1925) was a German mathematician and mathematics educator, known for his work in group theory, complex analysis, non-Euclidean geometry, and on the connections between geometry and group theory. His 1872 Erlangen Program, classifying geometries by their underlying symmetry groups, was a hugely influential synthesis of much of the mathematics of the day. In 1893 in Chicago, Klein was a keynote speaker at the International Mathematical Congress held as part of the World's Columbian Exposition. He supported that the University of Göttingen began admitting women in 1893. He supervised the first Ph.D. thesis in mathematics written at Göttingen by Grace Chisholm Young, the first woman to receive a doctorate in any field in Germany.

Around 1900, Klein began to take an interest in mathematical instruction in schools. In 1905, he played a decisive role in formulating a plan recommending that analytic geometry, the rudiments of differential and integral calculus, and the function concept be taught in secondary schools. This recommendation was gradually implemented in many countries around the world.

In 1908, Klein was elected the first president of the International Commission on Mathematical Instruction ICMI, founded at the Rome International Congress of Mathematicians. Under his guidance, the German branch of the Commission published many volumes on the teaching of mathematics at all levels in Germany. ICME-13 has devoted an entire open access monograph to his legacy (see https://www.springer.com/gp/book/9783319993850).

The two volumes of Klein’s landmark book titled Elementary mathematics from a higher standpoint (Arithmetic, Algebra, Analysis and Geometry) were publish in 2016 with a new translation into English. The third volume titled Precision Mathematics and Approximation Mathematics was translated into English in 2016 for the first time.