The aim of this paper is to show, by way of two case studies, the value of including Human Factors in interaction and interface design specification. It is argued that Human Factors offers and unique and useful perspective and contributes positively to design. Human Factors sits between subject matter experts and software engineers, translating user requirements though the applications of theory, models and methods. This results in software design requirements that have been intelligently interpreted and presented in a graphical manner. The two case studies demonstrate the differences between the interfaces with and without Human Factors input. Both cases show quantitative and qualitative benefits of including Human Factors in design. Performance improvements between 20-70 percent were demonstrated, which is typical of Human Factors design interventions.