A Spin Density Wave (SDW) arises when spin-up and spin-down Fermi surfaces are nested by a wavevector Q = 2k_F: states at k can scatter to k+Q. This drives itinerant antiferromagnetism: m(x) = M·cos(Qx + φ) without localized moments. SDW gaps open only at nested Fermi surface sections, unlike charge-density waves. Famous examples: chromium metal (T_SDW = 311 K) and organic conductors. The order parameter couples spin-up electrons at k to spin-down electrons at k+Q.