Apoptosis, or programmed cell death, is a physiological form of cell death that 
plays a critical role in the development and maintenance of multicellular 
organisms. Apoptosis is characterized based on morphological and biochemical 
criteria. Morphological characteristics include cell shrinkage, cytoplasmic 
condensation, chromatin segregation and condensation, membrane blebbing, and the 
formation of membrane-bound apoptotic bodies, whereas the biochemical hallmark 
of apoptosis is internucleosomal DNA cleavage into oligonucleosome-length 
fragments. A great deal of research is aimed at defining the molecular 
mechanisms that play a role in apoptosis. As one of the common end points of 
experiments related to apoptosis is in fact the death of the cell, it has become 
important to develop reliable assays to measure cell death that may be compared 
among the various systems being investigated. This chapter reviews many of the 
current methods used to measure apoptotic cell death and points out strengths 
and weaknesses of each approach with respect to the system being examined and 
the questions being asked. Traditional cell-based methods, including light and 
electron microscopy, vital dyes, and nuclear stains, are described. Biochemical 
methods such as DNA laddering, lactate dehydrogenase enzyme release, and MTT/XTT 
enzyme activity are described as well. Additionally, terminal deoxynucleotidyl 
transferase-mediated dUTP-biotin nick end labeling of DNA fragments (TUNEL) and 
in situ end labeling (ISEL) techniques are reviewed, which when used in 
conjunction with standard flow cytometric staining methods may yield informative 
data relating cell death to various cellular parameters, including cell cycle 
and cell phenotype. The use of one or more of the methods described in this 
chapter for measuring cell death should enable investigators to accurately 
assess apoptosis in the context of the various models being examined and help 
define causal relationships between the mechanisms that regulate apoptosis and 
the cell death event itself.