Abstract: Catholic theology traditionally speaks of Purgatory, a state experienced by souls after death who are ultimately destined to enjoy the presence of God in heaven for eternity, but must first undergo some kind of purification. This dissertation shows how the church's doctrinal definitions on purgatory are best understood if the experience of purgation has both penal ('detention') and non-penal yet purifying ('refinement') elements. Particular attention is paid to the saying of Jesus 'You will not get out until you have paid the last penny' (Mt 5:26, 18:23-35; Lk 12:59): although this is not a traditional locus for teaching on Purgatory, there is nothing in tradition to prevent this interpretation, and the tools of Biblical criticism suggest that the most natural reading is that Jesus is indeed teaching about a post-mortem state of finite duration.