Understanding the potential adverse health effects of our natural environment is of the greatest importance for the development of national and international programs to protect public health. Natural earth processes, including dust generation and mobilization, earthquakes, landslides, volcanic emissions, etc., continue to cause numerous deaths and immense suffering worldwide. Traditionally, the limited extent of interdisciplinary cooperation at the interface between earth sciences and public health has restricted the ability of scientists to respond to complex environmental health problems. Medical Geology is aimed at improving interdisciplinary interactions among earth and public health scientists, providing the basis for innovative and exciting research that can lead to new discoveries and greater knowledge. There is a need for a common outlet for medical geology publications to facilitate communications between different disciplines and different countries. It is with this objective in mind that we agreed to produce two special issues for the readers of Environmental Geochemistry and Health. The first issue (Vol. 29, no. 2) was published in April 2007. With this second issue, we have brought together more in the series of papers dealing with some of the more important medical geology issues with emphasis on the impacts in developing countries. Medical Geology will continue to make significant contributions to the study of epidemiology and public health, provided that the lines of collaboration between geologists, epidemiologists, toxicologists, clinicians and public health professionals are kept open. This Special Issue of Environmental Geochemistry and Health is aimed at strengthening these lines of communication by demonstrating the importance of geology in human health and disease. In the first paper in this volume, Shao and Zheng have presented the results of investigating the common rumour that Marco Polo’s horses died because of selenium poisoning in China. The results are quite intriguing. Oruc descibes the problems of fluoride and drinking water in Turkey. Davies describes the environmental health impacts of East African Rift volcanism. Monteil and others have looked into the effects of natural dust in the Caribbean derived from the Sahara desert. This is a global issue because the dust from the large deserts in Africa, Asia and Australia affects the whole globe. Arsenic and fluoride are of global concern, increasing all the time, and Armienta and Segovia describe arsenic and fluoride in the groundwater of Mexico and the effects Guest editors Olle Selinus: Geological Survey of Sweden, Uppsala 751 28, Sweden Robert B. Finkelman: University of Texas at Dallas, Department of Geosciences, Richardson, TX 750833 Jose A. Centeno: U.S. Armed Forces Institute of Pathology, Washington, DC 20306-6000 USA