Literature review examining international definitions and measures of crowding, with the aim of establishing a clear definition of crowding to be used in New Zealand and summarising research on the harmful effects of crowding. Commissioned in response to government and public concerns about the impacts on health due to crowding. The differentiation between the objective term ‘density’ and subjective term ‘crowding’ is discussed – noting that ‘crowding’ is a largely subjective term determined by either policy makers and statisticians and is often described as an environment which results in an adverse psychological response to density. The rationales behind regulatory and administrative definitions of crowding are discussed and it is deemed difficult to establish whether the standards are based on “beliefs about health, space, privacy, safety, lifestyle or morality”. Review asserts that it is virtually impossible to separate confounding factors such as nutrition and poverty from the effects of crowding. Variables such as social class, culture and age are difficult to rule out. And, while correlation between disease and ill-health due to overcrowding has been established by some studies, causation has not. The report concludes that more research is required, and a list of research topic recommendations is provided within a research agenda.