Generalized Parton Distributions (GPDs) are key to a 3D tomography of hadrons and nuclei, as they provide information on the spatial distribution of quarks and gluons. The complexity of GPDs results in unique challenges in their calculation using QCD based approaches and in their extraction from data. For example, GPDs appear in the QCD factorization of the cross-section associated with deeply virtual Compton Scattering, however, to obtain the GPDs an inverse problem must be solved which has been shown to have multiple solutions. In this talk we will highlight some of the challenges in the calculation and extraction from data of the GPDs, and provide some recent results.
