Ladder logic is a programming language that is used to program a PLC (Programmable Logic Controller). It is a graphical PLC programming language which expresses logic operations with symbolic notation using ladder diagrams, much like the rails and rungs of a traditional relay logic circuit. It is used by engineers and electricians to execute logical, sequential, counting, timing and arithmetic tasks in order to carry industrial automation applications.

In the good old days, machine and process automation was accomplished using a hard wired control system known as relay logic. With the advent of microprocessors and the invention of the PLC, relay logic quickly became superseded by programming languages such as ladder logic.