It is perhaps easier to explain what theory is not rather than what it is. Theory is not facts or data. Nor is theory a hypothesis, or a case study. It is not a literature review. A theory is a set of general principles or ideas that are meant to explain how something works, and is independent of what it intends to explain. The purpose of a theory (or set of theories) is to help explain what causes something to occur, or to inform us of the likely consequences of a phenomenon. In so doing, theories can be more or less abstract, and be pitched at different levels - explaining society, processes, relations, behaviour and perceptions. For practitioners, theories can enhance understanding and inform decision-making. For researchers, theories shape the framing of their data, and are often presented as an essential part of any well-designed research project. Reflecting this, Hambrick (2007: 1346) argues that theory is essential for a field to flourish and advance. Indeed, many management journals require scholars to make a ‘theoretical contribution’ to get published, prompting something of an obsession with a theory-driven approach in management-related areas. Thus, while recognizing the value and importance of theory, we offer a cautionary note. Specifically, we suggest that it may be fruitful for a field to support initial consideration of phenomena-driven trends or patterns before becoming fixated on having a theoretical explanation. For example, that smoking can cause harm and ill health in humans does not need a theory to prove its validity (Hambrick, 2007). Reflecting this, in disciplines such as sociology, economics and finance there has been less of an ‘essential need’ to publish with some new theoretical development in mind. Instead, ideas, logics, concepts, premises are given due attention and the notion of exploring data is seen as valid and valuable in deciding if certain issues or phenomena are in themselves evident or emergent. Where this is the case, theory can then help to understand and explain such issues. Theory is therefore a crucial lens on the world, one that provides value in addressing both evident and emergent issues. Notwithstanding that empirics and theory both contribute value and vibrancy to a field, our focus here is on the role of theory, and some of the specific theories used in employment relations (ER) and human resource management (HRM) research.