This book is the first to address both the theoretical and practical issues which arise when describing the geographical distribution of disease and investigating apparent disease clusters. Requirements in terms of population data, disease incidence and mortality are considered and related to the scale at which a study is being carried out. Statistical methods are reviewed for large scale correlation studies, intermediate scale smoothing exercises, and small scale clustering investigations, and the problems of interpretation are discussed. Problems of measuring environmental exposures at different scales are also reviewed. These issues are then related to current practice via a comprehensive set of case studies which include a large correlation study in China, clustering of asthma attacks, the Sellafield-leukemia cluster, environmental clusters of mesothelioma in Turkey, the investigation of disease following the industrial accident at Sevesco, and a multi-source study of cancer incidence around incinerators.