In the last century, the average life expectancy at birth increased from roughly 45 years in the early 1900s to more than 80 years of age at present. However, living longer is often related to different levels of frailty. There is no curative treatment for frailty-the interventions that have been described as effective to slow or delay the onset of frailty are physical activity and nutritional interventions. Maintaining adequate nutrition status is important to reduce the risk of chronic diseases, many of which are age-related. On the other hand, frailty itself may have a negative effect on eating and, thus, on the nutritional status. This Special Issue, "Nutrition, Diet Quality, Aging and Frailty", addresses the existing knowledge on nutrition regarding the causative factors of frailty and disease due to aging, i.e., strategies for delaying the pathological effects of aging. It consists of twelve peer-reviewed papers covering original research, protocol development, methodological studies, narrative or systematic reviews, and meta-analyses, to better understand these complex relationships.