The integration of frailty measures in clinical practice is crucial for the development of interventions against disabling conditions in older persons. The frailty phenotype (proposed and validated by Fried and colleagues in the Cardiovascular Health Study) and the Frailty Index (proposed and validated by Rockwood and colleagues in the Canadian Study of Health and Aging) represent the most known operational definitions of frailty in older persons. Unfortunately, they are often wrongly considered as alternatives and/or substitutables. These two instruments are indeed very different and should rather be considered as complementary. In the present paper, we discuss about the designs and rationals of the two instruments, proposing the correct ways for having them implemented in the clinical setting.