Aging is associated with reduced muscle mass, metabolic impairments, low cardiorespiratory fitness, and impaired physical function. Aging per se may account for some proportion of these declines; however, aging is associated with physical inactivity and lifestyle behaviors that also may contribute to vascular dysfunction and microvascular rarefaction. As the major interface between the circulation and skeletal muscle, the microvasculature can affect cardiometabolic and muscular health across the lifespan. For example, a reduction in skeletal muscle capillarization may contribute to aging-associated impairments in muscle mass, metabolism, fitness, and function through limited delivery of oxygen, amino acids, nutrients, and hormones (Fig. 1). In this article, we explore the hypothesis that capillary rarefaction contributes to these aging-associated declines, and preservation of skeletal muscle capillarization is essential for maintaining fitness and function across the lifespan. A number of studies have compared gastrocnemius or vastus lateralis muscle capillarization between young (20–30 yr) and older (> 60 yr) men and women, showing that older adults have 12%–25% lower capillarization than young adults (1–4). These results typically hold true for overall skeletal muscle and type II muscle fibers (1, 3, 4), whereas at least one study has shown a similar trend in type I muscle fibers (1). Although some studies report lower capillary density (number of capillaries per mm2 of muscle cross-sectional area [CSA]) in older adults (2), the majority of studies show that capillarization is lower in older adults when expressed as the number of capillary contacts (the average number of capillaries in contact with each muscle fiber) or individual capillary-to-fiber ratio (the number of whole capillary equivalents in contact with each muscle fiber)(1, 3, 4), indicating that the number of capillaries is lower in muscle from older adults. It should be noted that not all studies have identified significant decreases in skeletal muscle capillarization in older adults. For example, Chilibeck et al.(5) found no statistically significant difference in capillary-to-fiber ratio between young and older adults who were recreationally active. Studies