Spin-polarized metastable atom deexcitation spectroscopy (SPMDS) demonstrates the ultimate surface sensitivity for measuring electron spin polarizations. We firstly explain the principles and the technical aspects to detect surface electron spins with spin-polarized metastable He atom beams. We then apply SPMDS to the studies on the spin polarization of Na- and oxygen-adsorbed Fe(100) surfaces, the spin polarization induced in copper phthalocyanine (CuPc) molecules, and the magnetism of perpendicularly magnetized Fe ultrathin films on Cu(100) substrates.