Few people appreciate the insidious nature of asynchronous events until they encounter a problem that is difficult to reproduce. This chapter discusses signals and their effect on processes, emphasizing the concurrent aspects of signal handling. The chapter begins by defining basic signal concepts such as generation and delivery as well as explaining the difference between ignoring a signal and blocking a signal. Sample programs demonstrate how to use signals for notification and how to suspend a process while waiting for a signal. The chapter also covers error handling, signal safety and asynchronous I/O.