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This article is part of an analysis which deals with practitioners psychologists who are working in institutional systems meant for professional integration. It is observed that with regard to their commitments as well as to their actions, these institutional systems are based on the concepts of skills and habilities. For this reason, we ask about both the situations that require the actions of psychologists and how the last ones understand the concepts they use.From a theorical point of view, we first of all speak about the social context in which arise the concept of skills and habilities and about the consequencies of using it. And then, we consider this knowledge as an evaluative or ideological knowledge (Beauvois, 1994). This point leads us to see the institutions and their psychologists as ideological and orthodox systems (Deconchy, 1980). These systems create inwardly some socio cognitive mechanisms in order to protect all the knowledges against the attacks from the exterior. Two kinds of research allow us to verify this hypothesis. It turns thus out that practitioners psychologists install defence mechanisms of the type « social regulation » aimed at compensating the rational fragility of the knowledges on which they base their actions. In a theoretical view, these researchs allow us to consider the use of Deconchy’s model with a different population of the one that permit his experimental validation.