In this article the authors examine six different analyses of the meaning of unless that have been argued in the literature, and present a new analysis in which a careful distinction is made between the semantic meaning and the pragmatic interpretations of unless. Contrary to the common belief that unless cannot be used in irrealis conditionals, it is shown that unless can actually be used in two different senses there, one in which it alternates with except if, but not if . . . not (e.g. He wouldn't have done it {unless I had / except if I had / *if I hadn't} asked him to, and I didn't) and one in which it alternates with if . . . not but not with except if (e.g. You did do well, but you wouldn't have succeeded {unless I had / if I hadn't / *except if I had} helped you). It is shown, finally, how the various interpretations of unless can be reconciled with a common semantic meaning and how the different constraints on the use of unless, except if and if . . . not can be accounted for.