Systems theory has long informed theoretical developments in employment relations and HRM. There is a rich and interdisciplinary underpinning to systems logic stretching back to classic research in work and organisations (Burns and Stalker, 1961), Dunlop’s (1958) Industrial Relations Systems, and foundational organisation theory (Katz and Kahn, 1966). Today, systems theory is still frequently theoretically invoked, if less empirically employed. This chapter will present an overview of systems theory and its purpose before tracing its historical antecedents and key domain assumptions. The chapter will then review some valuable applications of system thinking in employment relations and HRM, before evaluating limitations and future prospects. Overall, the chapter surfaces the long standing tensions between the intuitive appeal of systems logic and difficulty surrounding its application. The chapter concludes that, despite these challenges, the changing nature of work as characterized by fragmentation, flexibility, feminisation and finacialization (Rubery, 2015) provide a strong mandate for a system theory renaissance in HRM and employment relations.