This essay treats Leonard Euler and his work in mathematics. I begin my essay with a biography, where I write about the youth of Euler and his life in St Petersburg and Berlin. The Biography also treats Eulers family, friends and colleagues. I end the boigraphy with a summary of the works of Euler. In the second part of my essay, the mathematical part, I have choosen to write about continued fraction, Euler's theorem, the vibrating string and the Euler-characteristic. I have described the mathematical history of continued fractions and geometry, from the beginning and almost to our days. Some of the mathematical chapters are followed by examples. In the chapter Letter to a German princess, a textbook on physics and philosophy writtem by Euler, I've made a summary of the books, and made some personal opinions. In a sequel to this essay, I will study the history and mathematics of the Euler characteristic and related topics.