This chapter covers signals and signal processing. It presents a unified treatment of signals and interrupts, which helps put signals into a proper perspective. It treats signals as interrupts to processes, which diverts a process from its normal executions to do signal processing. It explains sources of signals, which include signals from hardware, exceptions and other processes. Then it uses examples to illustrate common usage of signals in Unix/Linux. It explains signal processing in Unix/Linux in detail. These include signal types, signal vector bits, signal mask bits, signal handlers in the process PROC structure and signal processing steps. It uses examples to show how to install signal catchers to handle program exceptions, such as segmentation faults, in user mode. It also discusses the suitability of using signals as a mechanism for Interprocess Communication (IPC). The programming project is for the reader to use signals and pipes to implement an IPC mechanism for processes to exchange message.