This publication, 'The Safe Isolation of Plant and Equipment - HSG 253', is a reference tool for duty holders in the onshore and offshore oil and gas industry, chemical manufacturing, and pipelines associated with these industries. It will help duty holders to develop, review and enhance their own isolation standards and procedures. It also has general application to all industries where process isolations are made and to mobile offshore drilling units where relevant. It provides guidance on how to isolate plant and equipment safely, and how to reduce the risk of releasing hazardous substances during intrusive activities such as maintenance and sampling operations. It also includes a methodology for selecting 'baseline' process isolation standards and outlines preventive and risk reduction measures.