This work is set within the extensive framework of the renewed studies on European nobility and its target is the complex relationship between the Castilian nobility and Catholic moral theology in the period that goes from 1550 to 1650. Its main purpose is to investigate the extent to which the Castilian nobles inhabited a moral universe where certain rules were applied with respect to what was morally legal; rules that generated many and constant doubts. The study thus focuses on the nobles and their activity in the world: activity that falls within some concrete and specific economic, social and political margins of the time, but also, and this is what we aim to put forward, moral margins. All of this within the context of the post-Tridentine Catholic Europe: increase in the frequency of confession, expansion of the examination of conscience, detailed analysis of sins, etc. By analysing the consultations made by the Castilian nobles to theologians, the work presents a group of nobles submitted to continuous uncertainty about practically all the aspects of their lives as lords and as individuals: the sale of offices on the manors, the payment of fair salaries to servants, the payment of debts, compliance with the legislation on luxury, the persecution of public sins, the non-shooting (game), etc. In these aspects and others, the modern noble is situated in a moral topography with well-defined contours that impose limits on their political, social, and economic action. The principles of natural law (not harming others, seeking good, distributive and commutative justice, superfluous goods, etc.), the maxims of positive divine right (thou shall not kill, thou shall not steal, etc.) and the positive human, civilian and canonical law (restitution, obligation in the knowledge of complying with human laws, etc.) are all interconnectable for the conscience of the nobles who must be subjected to frequent judgements in the tribunal of penance. The moral discourse is an ambivalent discourse for the Castilian nobles of the Modern Age, because, on the one hand, it legitimises their position, but, on the other hand, it sometimes restricts their capacity to act.