The last decade or so has seen the development of a comprehensive approach to the description of hadron and nuclear structure. This framework encodes our knowledge of hadrons and nuclei in the Wigner distributions of the fundamental constituents − quarks and gluons − whose interactions are governed by quantum chromodynamics (QCD). A Wigner distribution is a quantum mechanical concept analogous to the classical notion of a phase space distribution, and allows for a quark and gluon tomography of hadrons and nuclei. Wigner distributions provide a new mathematical structure within which some of the most important questions in hadron physics can be framed and addressed, for example: how is mass and energy distributed inside hadrons and nuclei; what are the effective degrees of freedom and their important correlations; how are orbital angular momentum and spin distributed among the quarks and gluons; and ultimately insight into the mechanisms that generate the proton mass and color confinement.

From the Wigner distributions it is possible to construct measurable quantities known as generalized parton distributions (GPDs) and transverse momentum-dependent partons distributions (TMDs), where GPDs are key to a spatial tomography of hadrons and TMDs allow for their momentum tomography. With the recent completion of the 12 GeV upgrade at Jefferson Lab, a new generation of experiments will soon provide a tremendous amount of new empirical data on nucleon and nuclear GPDs and TMDs. To help support and guide this Jefferson Lab program, and related programs at COMPASS, Fermilab, RHIC and e⁺e^- colliders such as BES, this INT program brought together experts on GPDs and TMDs, as well as key people in related fields, for extensive discussion and collaboration aimed at insuring the success of this ambitious new direction in hadron physics. This 5 week INT program, which included two week-long workshops, explored numerous aspects of hadron tomography/ imaging, for example:

• a clear articulation of why imaging will be transformational to our understanding of hadron and nuclear structure;
• identify new aspects of QCD and hadron structure that imaging can address;
• ideals on how imaging can shed light on the explicit role of quarks and gluons in the nucleon-nucleon interaction;
• how imaging can lead to a better understanding of confinement and the origin of mass;
• remaining challenges in experiment and theory that need to be overcome to develop a complete quark and gluon tomography of hadrons and nuclei;
• the impact of a tomography program on the LHC;
• identify what Jefferson Lab and other existing facilities can explore and what must be done at an EIC.

A key outcome of the program will be a white paper that reviews the topics above, and thereby provides guidance for the Jefferson Lab tomography program, and a road map towards a future imaging program at an electron-ion collider.