This chapter focuses on the Christian tradition of practical ethics in Europe of the High Middle Ages, here in relation to lay practice. Its particular focus is vows, voluntary commitments to God to undertake a good action, which were a popular feature of popular piety in this period. This was an ethical domain rich in complex rules. Typically, pious laypeople took vows of pilgrimage, prayer, fasting and chastity, either for a fixed period or in perpetuity, as a penitential act or as a spontaneous act of piety. Especially from the thirteenth century, the church established detailed rules intended to moderate such vows, given the ways in which they could conflict with other, ordinary obligations. The chapter takes as an example the pastoral rules relating to vows in four texts from thirteenth-century France and England, Thomas of Chobham, Peter the Chanter, Raymond of Penafort and John of Freiburg. These books of penitential advice shows that detailed rules about vows were devised by the clergy with the concerns of ordinary people in mind. In this light, casuistry – which would later become notorious as the archetype of ethical legalism – is revealed not so much as an invasion of private conscience, but an attempt to facilitate personal devotional projects.