The subjunctive mood has frequently been explained in terms of unreality, presupposition, non-assertion and the distinction between new and old information. Although these explanations offer a partial account of the semantics of this mood, it is shown that many occurrences of the subjunctive mood remain unexplained. This being so, the present paper aims at explaining the indicative and subjunctive mood in impersonal expressions with the verb ser from a Cognitive Grammar perspective of linguistic analysis. The analysis shows that the variation between the indicative and subjunctive mood in this grammatical context can be explained in terms of dominion and control. An extension of the analysis further shows that it may account for the occurrence of the subjunctive mood in other grammatical contexts.