Newton's life and work are briefly summarized and it is reasoned that we are in a better position today than a century ago for evaluating his accomplishment. ThePrincipiais then presented and discussed in some detail. Stress is laid especially on the difference between the first two books, where Newton put together into one coherent ‘general mechanics’ his own and his predecessors' discoveries, and the third, where the theorems are applied for explaining the Solar System, the motions of planets and comets, and the tides. There follows an explanation of how Newton's ideas were propagated, even though thePrincipia, unlike theOpticks, was understood by only a few scientists. Through the work of D. Bernoulli and L. Euler, especially, Newton's mechanics was transformed and expanded into an endeavour of endless application. It is shown that the theory of relativity, although marking a limit to the validity of Newton's mechanics, has made clear how much better than most of his critics Newton understood the problems behind his work.