The new century was opened by Gauss's Disquisitiones Arithmeticae of 1801, which brought that theory to essentially its modern state. Indeed, when Neil Sloane and I wanted to summarize the classi¯cation theory of binary forms for one of our books [3], we found that the only Number Theory textbook in the Cambridge Mathematical Library that handled every case was still the Disquisitiones! Gauss's initial exploration of ternary quadratics was continued by his great disciple Eisenstein, while Dirichlet started the analytic theory by his class number formula of 1839.