The importance of apoptosis has been increasingly appreciated during the last decade. Apoptosis and cell proliferation are two opposing and tightly-controlled events in physiology and pathology. In the 21st century, therapy through either enhancement or suppression of the apoptotic process, depending on the type of disease, will be one important task in medicine. Learning methods to document and quantify apoptotic events is the first step in embracing the approaching era in which modulation of apoptosis may become a goal of clinical treatment. In this article, the second part of two related reviews (first review, J Formos Med. Assoc. 1999; 98:381-93), characteristics of apoptosis, including morphology, biochemistry, and the efficient elimination of apoptotic cells by tissues are discussed with a special emphasis on the quantitative analyses of apoptosis in biomedical research.