Over the past centuries, the average life expectancy at birth has increased substantially. Unfortunately, a progressive decline in physiologic reserves inevitably occurs with ageing. Frailty results from reaching a threshold of decline across multiple systems. Deterioration of the musculoskeletal system, osteoporosis though mainly sarcopenia, has a major functional impact and predispose elderly individuals to disability, falls and fractures. No consensus exists on the conceptual and operational definitions of frailty and sarcopenia, and some potential determinants of musculoskeletal frailty have been less well established in men than in women. Therefore, the objectives of this doctoral project were to examine the concept of frailty and musculoskeletal frailty and to study the role of underinvestigated determinants of osteoporosis and sarcopenia in ageing men. More specifically, we aimed i) to clarify the concept of frailty as well as musculoskeletal frailty and its two components, sarcopenia and osteoporosis (chapter 1 and 2); ii) to examine the association between sarcopenia and osteoporosis in men and to examine whether sarcopenia is a determinant of male osteoporosis (chapter 3); iii) to study the role of sex hormones and vitamin D in the development of sarcopenia in ageing men (chapter 4) and iv) to study the role of bone turnover in the age-related bone loss in men (chapter 5). In chapters 3, 4 and 5 we used data from the Leuven and Manchester cohorts of the European Male Ageing Study (EMAS). EMAS is a population-based prospective study focusing primarily on the influence of hormonal decline on health and disease in ageing men. The baseline phase took place during 2003 and 2005, with 3400 men between 40-79 years being recruited from population registers in eight centres. The follow-up measurements were performed after a mean follow-up of 4.3 years. In Leuven and Manchester, 388 and 328 men respectively participated in the full study. The results of this doctoral project should contribute to the identification of men most at risk of developing osteoporosis and sarcopenia. Ultimately, it may contribute to a scientific basis for the development and validation of more effective strategies to prevent fractures and frailty in men.