This book connects two linguistic phenomena, modality and subordinators, so that both are seen in a new light, each adding to the understanding of the other. It argues that general subordinators (or complementizers) denote propositional modality (otherwise expressed by moods such as the indicative-subjunctive and epistemic-evidential modal markers). The book explores the hypothesis both on a cross-linguistic and on a language-branch specific level (the Germanic languages). One obvious connection between the indicative-subjunctive distinction and subordinators is that the former is typically manifested in subordinate clauses. Furthermore, both the indicative-subjunctive and subordinators determine clause types. More importantly, however, it is shown, through data from various languages, that subordinators themselves often denote the indicative-subjunctive distinction. In the Germanic languages, there is variation in many clause types between both the indicative and the subjunctive and that and if depending on the speaker's and/or the subject's certainty of the truth of the proposition.