The paper begins by defining some terms, and then refer to a number of technical and other difficulties. Finally it attempts to set out why risk assessment is important and what its purposes are. 2) First, risk and risk assessment - what are they?. 3) Risk is a subject of universal significance. Life is very uncertain, and we can achieve no object or benefit in it except by approaching nearer to particular hazards which lie between us and our objects. That approach represents acceptance of risk. 4) Risk assessment is a way of systematising our approach to hazard with a view to determining what is more and what is less risky. It helps us in the end to diminish our exposure while obtaining whatever benefits we have in mind, or to optimise the risks and the benefits.