Abstract: (The expresion of deontic and dynamic modality in La vida es sueño / Life is a dream by Pedro Calderón de la Barca) The difference between social rules, represented by the community, and the natural, uncensored impulses manifested by Segismundo before assuming that aun en sueños / no se pierde el hacer bien (Clotaldo, versos 1159-1160, Jornada Segunda ), states in the Calderón de la Barca’s play through root modal expressions - deontic and dynamic (among other strategies). The first refers to permitted or forbidden actions (equivalents to moral or social laws), and the second to what can be physically accomplished (equivalents to natural laws). The role of deontic modality is to restrict the sphere in which appear the dynamic modals: even if one might show an ability, a capacity to achieve an action, it can be restricted by the social conventions - by what has to be done. Dynamic and deontic can be expressed through the same verbs (as poder), situation which obliges the addressee to make interpretations related to the context (but the fact that the context itself can select both interpretations leads to misunderstandings between the characters). This ambiguity / polysemy specific to Spanish modal expressions can be found also in other languages, as we can see in the Romanian translations (by Sorin Mărculescu and Alexandru Popescu-Telega), which have to transfer both possibilities of interpretation through deontic or dynamic modality, as expressions of the moral dilemma which the characters have to face. The linguistic analysis of modal expressions contributes to a philosophical interpretation of this dramatic work.