Apoptosis, or programmed cell death, is activated in the course of successful anti-neoplastic therapy. Determining baseline levels of apoptosis and the increment of apoptosis induced by therapy can serve as useful prognostic markers. Thus, non-invasive assessment of apoptosis would be desirable to provide clinicians with information on therapeutic efficacy as well as for the development and testing of new anticancer drugs. In these regards, apoptosis detecting radio-probes (radiopharmaceuticals) have been extensively studied. Annexin A5 (annexin V) is an endogenous protein that binds with high affinity and specificity to phosphatidylserine, which is presented on the cell surface in an early process of apoptosis. Accordingly, apoptotic cells can be detected in vivo using annexin A5 labeled with radionuclides, such as 99mTc and 18F. To date, several annexin A5 radio-probes have been developed. Among these, 99mTc-HYNIC-annexin A5 is the best candidate for apoptosis imaging. The apoptosis imaging using radio-labeled annexin A5 has been applied for detecting apoptosis in vivo in the experimental and clinical evaluation of the tumor response to chemotherapy or radiotherapy. The present review describes apoptosis imaging with annexin A5 radio-probes, focusing on its application to the evaluation of the tumor response to chemotherapy. First, principles of apoptosis imaging with annexin A5 radio-probes are described. Next, experimental results with radio-labeled annexin A5 in the evaluation of therapeutic efficacy are discussed. Finally, clinical application of apoptosis imaging with radio-labeled annexin A5 is addressed.