[en] This Conceptual Design Report (CDR) presents the compelling scientific case for upgrading the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab to 12 GeV. Such a facility will make profound contributions to the study of hadronic matter. In particular, it will allow breakthrough programs to be launched in four main areas: (1) The experimental study of gluonic excitations in order to understand the funda- mentally new dynamics that underpins all of nuclear physics: the confinement of quarks. Theoretical conjectures, now strengthened by lattice QCD simulations, indicate that the most spectacular new prediction of QCD - quark confinement - occurs through the formation of a string-like 'flux tube' between quarks. This conclusion (and proposed mechanisms of flux tube formation) can be tested by determining the spectrum of the gluonic excitations of mesons. (2) The Fundamental Structure of the Nuclear Building Blocks. A vast improvement in our knowledge of the fundamental structure of the proton and neutron can be achieved. Not only can existing 'deep inelastic scattering' cross sections be extended for the first time to cover the critical region where their basic three-quark structure dominates, but also measure- ments of new 'deep exclusive scattering' cross sections will open the door to a comprehensive characterization of these wavefunctions using the framework of the Generalized Parton Distributions; these data will provide access to information on the correlations among the quarks. These studies will be complemented by detailed measurements of elastic and transition form factors, determining the dynamics underlying the quark-gluon structure through measurements of their high-momentum-transfer behavior and providing essential constraints on their description. (3) The Physics of Nuclei. A broad and diversified program of measurements that (taken together with the hadron studies outlined above) aims to provide a firm intellectual under-pinning for all of nuclear physics by answering the question 'How does the phenomenological description of nuclei as nucleons interacting via an effective interaction parameterized using meson exchange arise from the underlying dynamics of quarks and gluons?' It has two main components: The emergence of nuclei from QCD. Experiments aimed at understanding how the description of nuclei in terms of nucleons interacting via the N - N force arises from the more fundamental QCD description. It includes the investigation of the partonic structure of nuclei, of short range structures in nuclei, and of the modification of the quark-gluon structure of the nucleons and mesons by the nuclear environment. Fundamental QCD processes in the nuclear arena. Experiments aimed at understanding how hadron-hadron interactions arise from the underlying quark-gluon structure of QCD. (4) Tests of the Standard Model of electro-weak interactions and the determination of fundamental parameters of this model. Precision, parity-violating electron scattering experiments made feasible by the 12 GeV Upgrade have the sensitivity to search for deviations from the Standard Model that could signal the presence of new physics. Precision measurements of the two-photon decay widths and transition form factors of the three neutral pseudoscalar mesons π⁰, η, and η' via the Primakoff effect will lead to a significant improvement on our knowledge of chiral symmetry in QCD, in particular on the ratios of quark masses and on chiral anomalies. This science program has expanded significantly since the project was first presented to the Nuclear Sciences Advisory Committee (NSAC) in the form of a White Paper [WP01] produced as part of the 2001-2002 NSAC Long Range Planning Process and since the pre-Conceptual Design Report (pCDR) produced in June 2004 to document the Upgrade science and experimental equipment plans [pCDR]. While focusing on science, this document also provides a brief description of the required detector and accelerator upgrades so that it can serve as an overview of the entire plan for the 12 GeV project. This CDR was developed from the documentation presented in the pCDR [pCDR] and further discussions within the community since that document was released. We acknowledge here the many contributions of the entire JLab community, and note that the author list at the end of the pCDR includes the names of all contributors to the effort known to us. Many of them commented extensively on the earlier drafts, resulting in a much-improved document. This document would have been impossible without their intelligence, enthusiasm, time, and just plain hard work.