As programmed cell death (PCD) or apoptosis has emerged as an important regulator of development and homeostasis in multicellular organisms, methods to quantify apoptosis and to distinguish it from necrosis have been developed. This unit presents a set of assays for these purposes, many of which are technically very simple and ideally suited to the study of hematopoietic cells. The first basic protocol allows the qualitative and quantitative assessment of apoptosis in lymphocyte cell cultures using light or fluorescent microscopy. Three protocols follow that are designed to detect nuclear DNA fragmentation and support protocols describe methods to radiolabel the DNA and cytoplasm of the cells to be tested. Techniques that quantitate apoptotic cells using flow cytometry are then described and support protocols provide methods for priming T cell clones and freshly isolated lymph node cells, respectively, for T cell receptor (TCR)-induced apoptosis. Quantitative detection of DNA fragmentation in apoptotic cells is also described. TdT-mediated dUTP-biotin nick end-labeling (TUNEL) methods are provided for the detection of apoptotic cells, along with procedures for the flow cytometric quantitation of apoptotic cells using TUNEL, and TUNEL, staining of tissue sections to identify apoptotic cells. Since much remains incompletely understood about the molecular pathways of programmed death, and it is probably best to perform more than one of the basic protocols to confirm an observation of apoptotic cell death.