The openness of joint awareness between two or more subjects is a perceptual phenomenon. It involves a certain mutual awareness between the subjects, an awareness that makes reference to that very awareness itself. Properly characterized, such awareness can generate iterated awareness ‘x is aware that y is aware that x is aware...’ to whatever level the subjects can sustain. The openness should not be characterized in terms of Lewis–Schiffer common knowledge, the conditions for which are not met in many basic cases of joint attention. A range of phenomena, including linguistic communication and other interpersonal relations, that have previously been described in terms of common knowledge should rather be seen as involving open joint awareness. An Appendix to this chapter discusses the relations of this approach to Barwise's discussions, and disputes the claim that these mental phenomena require the postulation of self-involving situations.