All pathologic processes that produce disease are the result of injury to cells. It is cellular injury and the body’s reaction to cellular injury that produce disease. Cellular injury could be divided into the two subtypes, i.e., reversible cell injury and irreversible cell injury. The reversible cell injury includes swelling of cell and cell organelles and cell adaptation, i.e. hypertrophy, hyperplasia, atrophy and metaplasia. Hypertrophy refers to an increase in the size of cells and, with such change, an increase in the size of the organ. Hyperplasia constitutes an increase in the number of cells in an organ or tissue, which may then have increased volume. Shrinkage in the size of the cells by loss of cell substance is known as atrophy. Metaplasia is the replacement of one adult cell type by another adult cell type. The irreversible cell injury includes dense bodies within mitochondria and cell death. This cell death is due to one of two basic mechanisms: necrosis or apoptosis. Necrosis is the death of cells or tissues within a still-living individual. Apoptosis is a form of cell death in which individual cells literally destroy themselves and sometimes is refererred to as “programmed cell death” or “cell suicide”. It is important to understand the differences between these two types of cell death.