The emergence of public relations as an academic and practitioner discipline has benefited from input from many sources: social science, behavioural science, engineering, philosophy, business and management. One of the main theoretical underpinnings is systems theory, with parallels being drawn between systems and how communication is practised. Most public relations texts, however, including some of the most influential, do not do justice to the richness of systems theory, especially the most recent thinking on the subject. In this paper the author reviews the use of the better known systems theories, and examines some of the newer developments and applies these to the contemporary practice of public relations. The French school of thought called actor-network theory is also alluded to in order to extend the discussion. The paper is in three sections. The first focuses on the older systems theories and their traditional application to public relations practice as propounded in the prevalent Grunig and Hunt public relations models. The second looks at some of the newer models in systems thinking, notes how they differ from the older ones and examines how they have relevance to modern practice. In the third section the author uses two practical examples, the breaking down of organisational barriers and current and potential uses of the Internet, to demonstrate the inadequacies of the older systems approach and the applicability of the new theories.