Programmable logic controllers are microprocessor-based controllers which use programmable memory to store instructions and to implement functions such as logic, sequence, timing counting and arithmetic to control events. The study of these chips is increasingly being incorporated into engineering courses. This is a step-by-step approach to PLC theory and practice, using case studies from the author's own experience in industry. It offers comprehensive coverage of the subject, and includes diagrams, programming techniques and self-assessment questions.