At some stage in your programming life you may well have used a debugger, but did you wonder how it was able to step into and control your executable? In this talk we'll see how debuggers work by building one from scratch in a few lines of Go. In this talk Liz will explore how a debugger gains control of a process with the all-powerful ptrace system call. You'll see how we can find the machine code that corresponds to a line of human-readable source code and vice-versa. She will show how breakpoints are set, explain how the stack gets built up, and demonstrate how to generate a stack trace showing the path to that breakpoint. Even if you know nothing about machine code, you'll leave this talk with a better understanding of how a computer runs an executable, and how a debugger is able to start and stop the executable as you wish.