Drawing on the study of knowledge and action as a reference, this thesis explores how practitioners in two different firms perform their practice, 'knowing' and 'acting' simultaneously. It argues that types of knowledge, activities, individuals and context are interwoven at the moment of acting. However, this relationship is not static. The empirical work in a local labour placement company and in a multinational consultancy firm shows that practitioners resort to a host of different expressions of knowledge (or bundle of knowledge) when acting. Therefore, the prevailing role of explicit knowledge and the need for being a precedent in order to be applied is called into question.The empirical work also reveals that the bundle of knowledge is not static. It evolves over time and at the same time the prevailing type of knowledge varies depending on the type of practice and the practitioner's level of expertise. Therefore, the results underscore the fact that the relationship between knowledge and action is more dynamic and that both interplay simultaneously. Finally, this research shows that formal company categorisations (senior/junior) describe different practices rather than correspond to differential stocks of formal knowledge. This means that even when experts and novices apparently seem to be doing the same job, their actions are different as they are constituted through different combinations of knowledge types and orders of relevance. These results seem to point toward the fact that the essence of expertise resides in the expert's ability to reframe. He is able to reconstruct practice, whether by reframing his tasks or the overarching context.