Born at Nordfjordeid in Norway, Lie was a friend of Felix Klein, whose ideas influenced him. Among Lie's most important work is his founding of the theory of continuous groups, which are now called Lie groups in his honor. Another contribution was his discovery of contact transformations. On both these subjects Lie wrote major treatises. In 1886 he became professor of mathematics at Leipzig and in 1898 he returned to Norway to take up a post that had been instituted for him at the University of Kristiania. By now, however, his health was poor and he died in Kristiania the following year. Lie also did notable work on differential geometry and on the study of differential equations.