Estrogen and other sex steroid hormones can affect multiple functions of the vascular wall including production and activity of endothelium-derived factors, expression of adhesion molecules, contraction of the smooth muscle to adrenergic nerve stimulation and smooth muscle proliferation and migration. Effects on response of the smooth muscle may be associated with regulation of surface receptors, second messenger systems and contractile protein isoforms. Sex steroid hormones may act synergistically with other hormones to modulate vascular reactivity across the normal ovarian cycle and during pregnancy. Estrogen may also affect remodeling of the vascular wall through inhibition of smooth muscle cell proliferation, stimulation of cell migration and secretion of matrix proteins. It is becoming apparent that modulation of vascular function by sex steroid hormones involves complex interactions. Not all actions of estrogen can be explained by genomic regulation of protein synthesis. Continued progress in understanding how hormones modulate vascular function will depend, in part, on clearer interpretation of data relative to the particular conditions of each experiment. Important aspects to be considered include the effect of gender and vessel type on the distribution and function of hormone receptors present in the vessel wall; use of hormones within physiological concentration ranges; and more precise determination of the influence of hormone treatment duration whether acute, subacute or chronic.