Cell death is usually classified into two broad categories: apoptosis and necrosis. Necrosis is a passive, catabolic process, always pathological, that represents a cell's response to extreme accidental or toxic insults. Apoptosis, in contrast, occurs under normal physiological conditions and is an active process requiring energy. However, apoptosis can also be elicited in a pathological way by toxic injury or during disease processes. In these nonphysiological conditions, both types of cell death can be encountered following the same initial insult and the balance between death by apoptosis and by necrosis appears to depend upon the intensity of the injury and the level of available intracellular ATP. It is important, however, to discriminate between apoptosis and necrosis in pathological conditions, as therapeutic intervention could be considered in apoptotic cell death with putative new pharmacological agents aimed at interfering with the key molecular events involved. In most cases, none of the current laboratory techniques used alone allows for unambiguous identification of apoptotic cells. Some of the most common methods based on morphology, biochemistry, and plasma membrane changes are discussed in terms of specificity and possible sources of error in data interpretation. As a rule, classification of cell death in a given model should always include morphological examination coupled with at least one of the other assays.