The various definitions which have been proposed for death are stated and rejected. The author concludes that death and individuality are independent of each other and that death in unicellular and multicellular organisms is essentially similar. Death does not occur instantaneously. It is an irreversible loss of the property of being alive. A practical definition considers only the end phase of necrobiosis as death. An organism can be called dead only if and when all of its living has died. Death of living matter is not due to the destruction of a plasma membrane on its surface rendering it permeable to substances within the membrane. There is evidence that death is accompanied by a chemical decomposition of the substances forming protoplasm. The author holds that protein substances and some lipoids are combined with each other in living matter in what he calls "principal compounds" which are very unstable and decompose under the influence of different harmful effects. When these principal compounds of living matter become decomposed, life stops. This change is a chemical one and is not due to the annihilation or departure of any mysterious living force. The heat produced by the decomposition of 1 molecule of the principal compounds of living matter is greater than 40 Kilo calories, while the heat produced by the decomposition of 1 molecule of nitrogen chloride, one of the most unstable explosive substances, is only 38 Kilo calories. All agents which produce a chemical alteration of the component parts of the "principal compounds" of living matter bring about their decomposition and the destruction of the whole colloidal system of protoplasm.