This book investigates key security issues in connection with the physical layer for random wireless cellular networks. It first introduces readers to the fundamentals of information theoretic security in the physical layer. By examining recently introduced security techniques for wireless point-to-point communications, the book proposes new solutions to physical layer security based on shastic geometric frameworks for random cellular networks. It subsequently elaborates on physical-layer security in multi-tier heterogeneous networks. With the new modeled settings, the authors also verify the security performance with the impact of the full-duplex transceivers. The specific model design presented here offers a valuable point of reference for readers in related areas. In addition, the book highlights promising topics and proposes potential future research directions.