The elaboration and diversification of sexually selected weapons remain poorly understood. We argue that progress in this topic has been hindered by a strong bias in sexual selection research, and a tendency for weapons to be conflated with ornaments used in mate choice. Here, we outline how male–male competition and female choice are distinct mechanisms of sexual selection, and why weapons and ornaments are fundamentally different types of traits. We call for research on the factors contributing to weapon divergence, the potential for male–male competition to drive speciation, and the specific use of weapons in the context of direct fights versus displays. Given that weapons are first and foremost fighting structures, biomechanical approaches are an especially promising direction for understanding weapon design. The potential for male–male competition to drive the elaboration and diversification of weapon morphologies remains one of the most understudied topics in the field of sexual selection. Progress in our understanding of weapon evolution has been hampered by a strong bias in sexual selection research, and imprecise terminology that erroneously equates all sexually selected structures with ornaments used in mate choice. We outline how the processes and outcomes of female choice and male–male competition are distinct, and why weapons and ornaments are fundamentally different types of traits. We encourage an integration of engineering techniques into studies of weapon function and performance, and highlight examples of how this biomechanical approach promises to improve our understanding of the evolution of weapon design.