Epistemology is the branch of philosophy that studies the possibility of knowledge. In this chapter we show how understanding epistemology is essential for understanding the different ways in which knowledge is developed in organization studies. For that purpose, we begin by introducing philosophical foundations of epistemology and explain why and how knowledge is defined as ‘justified true belief’. We then go on to analyse the assumptions on which different epistemic systems within organization studies are based. Following this analysis, we build on constructivist philosophy to address the incommensurability of different epistemic systems.