The Fukushima Daiichi nuclear disaster released massive amounts of radioactive substances into the environment. For isotopes with short half-lives, the primary safety concerns relate to the impacts on human health by initial exposure relatively soon after the disaster. However, isotopes with long half-lives remain in the environment for a long time; they move through the environment by many pathways and raise concerns regarding the threats to human health and everyday life. To reduce these impacts from the radioactive substances that have been dispersed widely throughout the environment, it is necessary to remove and isolate these substances from the places where people live and work, which is the purpose of decontamination. This chapter discusses the concept of decontamination, the methods used in decontamination, the institutional frameworks for decontamination and the processing and disposal of the soil and waste that is generated by decontamination.