Providing an introduction, the scientific background, case studies and future perspectives of in-situ arsenic remediation technologies for soils, soil water and groundwater at geogenic and anthropogenic contaminated sites. The case studies present in-situ technologies about natural arsenic, specifically arsenate and arsenite, but also about organic arsenic compounds. This work covers geochemical, microbiological and plant ecological solutions for arsenic remediation. It will serve as a standard textbook for (post-)graduate students and researchers in the field of Environmental Sciences and Hydrogeochemistry as well as researchers, engineers, environmental scientists and chemists, toxicologists, medical scientists and even for general public seeking an in-depth view of arsenic which had been classed as a carcinogen. This book aims to stimulate awareness among administrators, policy makers and company executives of in-situ remediation technologies at sites contamined by arsenic and to improve the international cooperation on the subject.