In this paper, concept formation and agency are analyzed in critical encounters between home care workers and their elderly clients. A critical encounter is an event in which two or more relevant actors come together to deal with a problem that represents both a potentially shared object and a conflict of motives. There is both complementarity and tension betweeen the actors. To resolve the problem, the actors use mediating artifacts to take agentive action and to make attempts at conceptualizing the situation. A conceptualization attempt is an articulation of an idea that has integrative potential for establishing a perspective for a solution to the problem or conflict of motives. We present a novel model of the critical encounter and test it in an analysis of 26 home care visits in which a new tool called “mobility agreement” was implemented as a second stimulus. With the implementation of the mobility agreement, a new collective concept of sustainable mobility has begun to emerge. We analyze the discursive and enactive dynamics of the critical encounters which engender – or thwart – the formation of the concept of sustainable mobility.