When Leonhard Euler arrived at the Russian Academy of Sciences, at the age of 20, his career was supported and promoted by the Academy's secretary, the Prussian jurist and amateur mathematician Christian Goldbach (1690-1764). Their encounter matured into a lifelong friendship, as witnessed by nearly 200 letters sent over 35 years. This exchange - Euler's most substantial long-term correspondence - has now been edited for the first time with an English...