With the aging of the Canadian population, functional autonomy and quality of life among seniors are now important public health issues. We hypothesized that nutrition is an important determinant of the quality of aging because of its potential to modulate the transitions from vulnerability to frailty and dependence. Over the past 15 years, our research program addressed the prevalence, the determinants, and the consequences of undernutrition among seniors, especially the free-living frail elderly. Very low energy and nutrient intakes were observed as well as a high prevalence of involuntary weight loss. These chronic conditions were associated with early institutionalization and increased mortality rates. Intervention strategies were then developed and evaluated, including the Nutrition Screening Program and the Nutrition Support Program. The effectiveness of these programs was shown with respect to improvement of nutritional status. However, this improvement was not sufficient to produce significant changes in functional autonomy or quality of life. Methodological issues related to the conduct of intervention studies in this specific population were addressed. A conceptual framework of nutritional intervention is currently being validated. A large longitudinal study that is being undertaken will further contribute to our understanding of the aging process as determined by a modifiable factor such as nutrition.