Despite the introduction of new effective antiemetics, nausea and vomiting (emesis) remain troublesome side effects of chemotherapy in cancer patients. These side effects not only affect patients' quality of life adversely, but also reduce patient adherence to the chemotherapy treatment and may ultimately negatively affect disease progression. Since, there is no single antiemetic agent that is effective all the time, there is a need for better understanding of the biological nature of nausea and emesis in order to enhance pharmacological interventions, as well as a need for better psychological understanding so that effective behavioral interventions may also be developed. This review focuses on both physiological and psychological origins of nausea and vomiting as side effects of chemotherapy.