Meeting the ever-increasing needs for future wireless applications—such as ultra-high data rates, ultra-wide radio coverage, ultra-large device counts, and ultra-low latency—will be made possible in large part by the fifth generation (5G) network. In the context of the 5G network, where wireless signals are intrinsically susceptible to security breaches, security is a crucial topic that this article addresses. In particular, we concentrate on physical layer security, which uses the disruptive technologies to 5G to its advantage while protecting data confidentiality by taking use of the inherent randomness of the communications medium. Three of the most promising technologies are discussed: millimetre wave, huge multiple-input multiple-output, and heterogeneous networks. Based on the fundamental ideas behind each technology, we pinpoint the vast opportunities and enduring difficulties that security designers need to overcome. It is anticipated that this identification will significantly increase our understanding of physical layer security in the future.