Recent years have been marked by an enormous growth of wireless communication networks and an extensive use of wireless applications. In return, this phenomenal expansion is inducing more concerns about the privacy and the security of the users. For many years, the security challenge has been mainly addressed at the application layer using cryptographic techniques. However, with the emergence of ad-hoc and decentralised networks and the deployment of 5G and beyond wireless communication systems, the need for less complex securing techniques had become a necessity. It is mainly for this reason that wireless physical layer security has gained much attention from the research community. What distinguishes information-theoretic security compared to other high-layer cryptographic techniques is that it exploits the randomness and the fluctuations of the wireless channel to achieve security at a remarkably reduced computational complexity. However, these technical virtues rely heavily on perhaps idealistic channel state information assumptions. In this chapter, we look at the physical layer security paradigm from the channel uncertainty perspective. In particular, we discuss the ergodic secrecy capacity of wiretap channels when the transmitter is hampered by the imperfect knowledge of the channel state information (CSI).