Organizational knowing is fundamentally a collective endeavour through which heterogeneous materials and entities, such as ideas, concepts, artifacts, texts, persons, norms, and traditions are mobilized, modified, translated, distorted, exposed, used, ignored or hidden in view of some practical accomplishment, such as safety in a construction site. Safety as a form of organizational expertise is therefore situated in the system of ongoing practices, has both explicit and tacit dimensions, is relational and mediated by artifacts, that is, it is material as well as mental and representational. Using examples derived from the observation data we will discuss how safety-related knowledge is constituted, institutionalized, and continually redefined and renegotiated within the organizing process through the interplay between action and reflexivity.