Our aim is to investigate certain aspects of scepticism concerning dreaming (hereafter termed dream scepticism). Central to this enterprise is the dream hypothesis, viz. the hypothesis that one is asleep and dreaming rather than awake. Dream scepticism involves the claim that with respect to any given person, at any given time, it is possible that the dream hypothesis is true. This claim is typically aligned with what may be termed the indistinguishability thesis, the claim that dreams and waking experience are intrinsically or qualitatively indistinguishable.