This volume is the first English edition, with commentaries and introductory material, of the writings on spherical geometry by Leonhard Euler, Joseph-Louis Lagrange and Johann Heinrich Lambert. The book is divided into two parts. The first part contains essays related the works by Euler, Lagrange and Lambert on spherical geometry. The goal of these essays is to include these works in the appropriate mathematical, historical and philosophical contexts. The second part of the volume is devoted to the English translations of the original memoirs, originally written in Latin, French and German. These translations are new and they are done by mathematicians who are involved in the subjects of the memoirs. Footnotes by the editors and the translators are appended to the translations. The book is addressed to students and researchers in geometry. It constitutes an invaluable reference in mathematics, and also in the history and philosophy of mathematics.