This paper focuses on the developmental tendencies and mechanisms underlying the unfolding of mood systems in Romance complement clauses. In view of the fact that the subsequent dynamics of change can be better understood and motivated against the backdrop of the Latin system, we take the basic structure of the Latin mood system as the reference and necessary starting point of our analysis. After briefly discussing the basic approaches to the mechanisms of mood change in the relevant research literature that puts forward notions like ‘modal harmony’, ‘regrammation’, ‘lexicalization’, and ‘conventionalization’, the article develops a modal–semantic perspective that casts a different light on the convergent and divergent developments of mood in the complement clause domain of Romance languages. The modal–semantic approach allows, apart from a coherent description and analysis of the developments, recasting the question of whether mood, especially the subjunctive, also comes with its own semantic value(s) in complement clauses. This modal–semantic approach not only provides a coherent description and analysis of the developments but also allows for a re-examination of the abstract semantics of the subjunctive mood (in complement clauses), spelling out its basic semantic features.