Increased peripheral vascular resistance is considered the hallmark of hypertension (1). This increased vascular resistance is chiefly determined at the microvascular level, in particular at the small arteries and precapillary arterioles, and it is likely to reflect functional and structural changes of these vessels (2) (Table 1). Functional abnormalities of microvessels in established essential hypertension include reduced capillary blood flow under basal condition (3) and reduced vasodilatation in response to heating (5). Structural abnormalities in vessels of hypertensive patients include decreased luminal diameter, predominantly in large arterioles (5, 6), but also in capillaries (7), and a reduction in the density of vessels per volume of tissue (rarefaction), which occurs predominantly in the smallest vessels (7–11). Both can be viewed as a reflection of increased microvascular tone.