Introduction Individuals who excel in mathematics have always enjoyed a well deserved high reputation. Nevertheless, a few hundred years back, as an honourable occupation with means to social advancement, such an individual would need a patron in order to sustain the creative activities over a long period. Leonhard Euler (1707-1783) had the fortune of being supported successively by Peter the Great (1672-1725), Frederich the Great (1712- 1786) and the Great Empress Catherine (1729-1791), enabling him to become the leading mathematician who dominated much of the eighteenth century. In this note celebrating his tercentenary, I shall mention his work in number theory which extended over some fifty years. Although it makes up only a small part of his immense scientific output (it occupies only four volumes out of more than seventy of his complete work) it is mostly through his research in number theory that he will be remembered as a mathematician, and it is clear that arithmetic gave him the most satisfaction and also much frustration. Gazette readers will be familiar with many of his results which are very well explained in H. Davenport's famous text [1], and those who want to know more about the historic background, together with the rest of the subject matter itself, should consult A. Weil's definitive scholarly work [2], on which much of what I write is based. Some of the topics being mentioned here are also set out in Euler's own Introductio in analysin infinitorum (1748), which has now been translated into English [3]. Number theory, as a branch of mathematics concerned with the properties of whole numbers, can be said to date from the discoveries of Fermât (1601-1665). There is little doubt that he had proofs for many of the results discovered by him, but he did not publish them and was content with private communications with other interested scientists. It was thus left to Euler to set out the proofs for the mathematical community, and it will not be out of place here to quote G. H. Hardy: 'In number theory, proof is everything!' Euler took up Fermat's writing in 1730 and found many interesting statements concerning primes and sums of squares. As Weil [2] put it, 'He had discovered a topic which was to haunt him all his life.' Besides admiration from Lagrange (1736-1813) and Goldbach (1690-1764), there was not much enthusiasm for Euler's research in arithmetic among fellow scientists. Even his old friend Daniel Bernoulli (1700-1782) sometimes spoke disparagingly about such work - for example, when replying to a letter from Nicolas Fuss (1755-1826) reporting on what Euler had discovered, his less than enthusiastic reply might be paraphrased as 'So what! Why does the great man pay so much attention to prime numbers? Personally I value more your research into the strength of beams.' In the following I shall, of course, make use of the congruence notation introduced by Gauss (1777-1855). However, readers should remember that