This chapter discusses Karl Popper's celebrated criterion of falsifiability, his criterion of demarcation between empirical science on the one hand and pseudo-science and metaphysics on the other. It suggests that this criterion, far from being a mere proposal or convention, as Popper himself sometimes described it, is a natural outcome of the idea that empirical investigation plays an indispensable part in our acquisition of knowledge about the world. If falsifiability is to provide necessary and sufficient conditions for anything, it had better be a criterion of demarcation between what has empirical content and what does not have empirical content. The falsifiability criterion of demarcation, which forms the backbone of Popper's philosophy of science, has been severely criticized both at a general level and with regard to detail. The chapter emphasizes the significance of the change of perspective that Popper offers us, embodied in 'the view that a hypothesis can only be empirically tested.