Apoptosis is a physiological process that occurs in cells during development and normal cellular processes. The useless, unwanted, or damaged cells die during the apoptotic process. However, if signals instructing cells to carry out apoptosis are lost, a variety of malignant disorders may result. Normal mammary gland development is controlled by a balance between cell proliferation and apoptosis, and the balance is important for normal mammary gland differentiation. The formation of breast cancer will happen as soon as the balance is upset by carcinogens. The uncontrolled cell proliferation and the reduced levels of apoptosis will result in cancer growth. It is useful to detect apoptosis in breast cancer to determine the malignancy of the cancer. It is also important to measure the apoptosis index in breast cancer or other cancers treated with anticancer agents to verify the effect of the anticancer agent. In this chapter, we provide detailed protocols of commonly used apoptosis assays, including DNA ladder formation, enzyme-linked immunosorbent assay for histone/DNA fragment, poly-ADP-ribose-polymerase cleavage assay, and terminal deoxynucleotidyl transferase-mediated dUTP nick-end-labeling assay.