Based on the surviving Old English textual material, as well as on Old English dictionaries and the relevant literature, this work studies the role of preverbs (eg. Byrnan, ābyrnan, forbyrnan, gebyrnan, onbyrnan) as a transitivising mechanism under the scope of the Cardinal Transitivity approach. Focus is laid on Old English morphological causative pairs that show signs of lability, i.e. verbs that can function transitively or intransitively with no morphological marking. This work has two main objectives. On the one hand, to examine to what extent preverbs may influence the valence of verbs that are ambivalent from the point of view of their valence as well as to shed light on the effects preverbs may have on other parameters of transitivity such as telicity or affectedness. On the other hand, this book also explores a rather neglected topic so far: the interaction of preverbs and the Germanic morphological causative marker -jan as transitivising mechanisms in Old English.