The two East Baltic languages Lithuanian and Latvian feature a secondary mood of potentiality, known as the subjunctive that ultimately goes back to the supine, an infinitival form. Setting out with a general discussion of the historical relation between the two East Baltic languages, this book seeks to explain why, when and how the supine developed into the subjunctive. In doing so, it provides the first exhaustive account of the evolution of the morphosyntactic paradigm of the subjunctive and all its attested forms in the East Baltic dialects. It thereby contributes to our understanding of the emergence of morphosyntactic structures and the typology of infinitive-based verbal formations.