Abstract: Throughout the 160-year history of plasticity, concepts, principles, and methods based on the rigid-plastic assumption (such as the concept of the plastic limit state, the principle of limit analysis, and the slip-line field method) have played a crucial role and provided classical plastic solutions to a range of problems. However, traditional limit analysis contains an inherent paradox and is unsuitable for large-deformation analysis of structures and materials. Applying rigid-plasticity in the studies of energy-absorbing structures and novel materials requires careful review and revision of these principles and methods. When evaluating the errors of rigid-plastic solutions, the first step should be to construct the “best” rigid-plastic complete solutions. Based on this kind of “best” solutions, more accurate assessments and compensation for errors caused by neglecting elastic effects can be achieved. In some simpler problems, the concise analytical formulas derived from rigid-plastic analysis can guide engineering estimation and preliminary design, as well as help judge the rationality and accuracy of numerical simulation results.