The one governing power of the diocesan Bishop is described - in cann 381 and 391 - as being the ordinary, proper and immediate potestas regiminis expressed as legislative, executive and judicial power for the exercise of the Bishop's pastoral office (munus pastorale). This doctoral dissertation - prepared under supervision of Prof. Gianfranco Ghirlanda SJ - presents and discusses the two central characteristics of the diocesan Bishop's power of governance: its unity and threefold expression. Therefore, the thesis considers the key features of the potestas regiminis in CIC 1917, Vatican II and CIC 1983 (ch.1); can. 381 1 on the unity and characteristics of this power as exercised by the diocesan Bishop (ch 2); the three expressions of the power of governance - legislative, executive and judicial power - in general, in ca. 135 and in can. 391 (ch.3); and a selection of issues that enable a more extensive understanding of this governing power of the diocesan Bishop (ch.4). Because the potestas regiminis of the diocesan Bishop articulates important ecclesiological points related to the Church, the particular Church and the Episcopal office in the Church, this dissertation may be characterised as being both an exegetical (analytical-comparative) study of two canons and a canonical-ecclesiological study of the governance of the diocesan Bishop.