This chapter deals with the penalty of day fines in Spanish criminal law, examined both from the perspective of its legal regulation and its practical implementation. Particular attention is paid to explaining a remarkable peculiarity of the way in which day fines are applied in Spanish judicial practice: routinely lawyers, prosecutors and judges pay only superficial attention to the indications of the Spanish Penal Code as to how the daily fees of this penalty should be calculated. Hypothetically, the sole criterion for setting the daily unit should be the overall financial situation of the convicted, including not only his regular personal income but also property and other assets, debts, other sources of income, family obligations and other personal circumstances. However, in practice, in the vast majority of cases certain routine quantities are used through a method that is halfway between custom and intuition. Therefore, it can be said that theday fines system in Spain represents, instead of a way to take wealth seriously, rather one of taking income lightly