The development at ELI-NP of a new laser-based Inverse Compton Scattering gamma beam system, featuring extremely high intensities at very narrow bandwidths, opens new and important opportunities in nuclear science research. Nuclear photonics is undergoing a revival, the gamma beams with unprecedented features delivered at ELI-NP paving the way for high accuracy and detailed nuclear physics studies. A wide range of industrial, homeland security and healthcare applications will also experience an important boost. The combination of nuclear photonics with the technique of Nuclear Resonance Fluorescence (NRF) allows for the recovery of several physical quantities characterizing the excited nuclear states in a completely model independent way. These observables include the excitation energies, level widths, gamma decay branching ratios, spin quantum numbers, and parities. In the last decade, the NRF technique allowed for the discovery and detailed study of various phenomena in atomic nuclei. Examples are the collective magnetic dipole Scissors Mode in deformed nuclei, quadrupole excitations with mixed proton neutron symmetry, the electric Pygmy Dipole Resonance, octupole coupled excitations, or alpha-cluster states. The present Technical Design Report (TDR) deals with the application of the NRF technique at ELI-NP to study forefront nuclear structure research topics. The document presents some of the physics cases to be investigated and discusses the feasibility of the proposed experiments. The advanced characteristics of the gamma beams available at ELI-NP and the use of high efficiency detection systems will offer a powerful combination, unique in the world, for the investigation of the proposed physics cases. The main detection system for the NRF studies is a multi-detector array (ELIADE - ELI-NP Array of DEtectors) based on the use of composite high-purity Ge detectors and large volume LaBr3 scintillator detectors able to detect with high efficiency gamma rays with energies up to several MeV in the presence of the high radiation background produced by the gamma beams. Gamma-ray energies and angular distributions will be measured with high accuracy. The design of the array is made highly flexible to allow for an easy transposition in different locations in the high- and low-energy gamma beam areas, a fast change of configuration based on the needs of the experiments, the use of the detectors in other setups and easy maintenance to reduce the downtimes. NRF measurements will be possible starting from early stages of the Gamma Beam System operation at ELI-NP with both low- and high-energy gamma beams. Already in the initial phase of operation at low-energies below 3.5 MeV the gamma beams at ELI-NP will be competitive with the present state-of-the-art gamma beam systems