The concepts of tacit knowledge and tacit knowing have been of interest to philosophers and epistemologists as well as behavioral and social scientists. The tacit dimension can be found in both individual and collective practices in versatile, implicit, informal, and unintentional ways. There is no clear, broadly accepted definition of tacit knowledge, but rather its complexity and ambiguity are generally acknowledged. In this essay, Auli Toom begins by presenting some general definitions of tacit knowledge and knowing, and she then draws on four perspectives to construct a more thorough understanding of these concepts. According to Toom, tacit knowledge can be considered using either its philosophical foundations or psychological foundations. In addition, tacit knowledge can be understood as an accumulated product of thinking and action, and also as a process during action. Toom observes further that while tacit knowledge is perceived as an individual and personally accumulated knowledge base, its collective and organizational characteristics are widely recognized. Toom concludes by considering the situational and contextual nature of tacit knowledge. (Contains 76 footnotes.)