The role of interfacial interactions in determining the global response of joint systems is discussed in relation to the question of true interfacial failure and the ubiquitous occurrence of interphases. The discussion is from the point of view of systems involving adhesives, coatings and composites rather than, e.g., particle-particle or particle-substrate systems and is strictly conceptual in nature. It is proposed that interfacial interactions, rather than directly exerting an effect on the global response of joint systems, are instead the driving force for the many and varied processes that create interphases. It is such interphases, or transition zones between phases, which affect the global mechanical response of joint systems.