This paper presents a comparison between the concepts underlying 4D Trajectory Based Operations (TBOs) from the double viewpoint of NextGen and SESAR programs. In the proposed analysis, motivations justifying the introduction of 4D TBOs are presented first. After that, the different and similar technologies that are being applied to support 4D TBOs are discussed. This is followed by a discussion focused on the results obtained from different human-in-the-loop simulation activities. In addition, preliminary flight trials activities, planned and partly executed for concepts refinement and final validation, are also discussed. These validation activities, carried out on both NextGen and SESAR sides, aim to assess the impact of the aforementioned supporting technologies on both pilots and Air Traffic Controllers. This impact will actually be a key element for the effective implementation of 4D TBOs. Early benefits identified for SESAR and NextGen are described next. Finally, some comparisons and preliminary conclusions are presented.