Abstract: The present invention provides compounds, compositions and methods for inhibiting or reducing reactive oxygen species (ROS) production in cells, such as in cells of the vascular system and in particular the smooth muscle-containing vasculature and/or endothelial cell-containing vasculature and/or adventitial fibroblast-containing vasculature. ROS production may also be inhibited in non-vascular cells of animals including mammals such as humans. Non-vascular cells contemplated herein include nerve cells, stem cells, progenitor cells and some cancer and rumor cells. More particularly, the present invention provides agents and even more particularly, cell-impermeable agents, capable of modulating NADPH oxidase activity, function or levels, thereby controlling superoxide production and production of downstream ROS.