There is a common simplification in seismic design codes that independently computes the seismic force in the two orthogonal directions. Then, individual frames are designed based on the distributed lateral forces. The 3D effects, including the interaction of adjacent and orthogonal frames, are omitted in this process, while research shows that the design results differ between 2D and 3D methods. Although most seismic assessment procedures and philosophies have been developed for 2D frames, there is no standard prescription for adopting them to assess 3D structures. The progress in the computational capability of computers makes the 2D design philosophy not acceptable anymore, and the engineering community needs to move toward a 3D point of view in assessing and designing structures. Many works have investigated the 3D behavior of structures or proposed 3D design strategies for different issues. There is a need to clarify the importance of the problem as an independent research field and urge the researchers to shed light on other aspects of the 3D structural behavior. This paper reviews the research in which the 3D structural design or behavior is addressed and shows weaknesses and future research paths. The study includes four main parts: modeling, assessment, design, and optimization of 3D structures. Finally, the main results and recommendations are presented.