The genius of Carl Friedrich Gauss (1777-1855) and the novelty of his work (published in Latin, German, and occasionally French) in areas as diverse as number theory, probability and astronomy were already widely acknowledged during his lifetime. But it took another three generations of mathematicians to reveal the true extent of his output as they studied Gauss's extensive unpublished papers and his voluminous correspondence. This posthumous twelve-volume collection of Gauss's complete works, published between 1863 and 1933, marks the culmination of their efforts and provides a fascinating account of one of the great scientific minds of the nineteenth century. An invaluable source in tracing Gauss' development is his scientific diary, which was only discovered in 1898. Volume 10 Part I, published in 1917, contains a photographic reproduction of this manuscript and a detailed commentary, together with many letters about pure mathematics, and shorter works including a preface to Piazzi.