Scanning tunneling microscopy and spectroscopy have been applied to the study of the atomic structure and the insulator-metal transition of the alkali metal-adsorbed Si(111) surfaces. Tunneling images of the Na- and K-induced Si(111)3X1 surface, formed by metal deposition and simultaneous substrate annealing, showed that the 3X1 structure consisted of zigzag chains separated by a single atomic row. The K-induced 3X1 surface contained more vacancy defects, possibly because of the larger lattice mismatch between K and Si. Upon additional deposition onto the 3X1 surface kept at room temperature, a multilayer of Na and K was grown. The Na overlayer was highly ordered whereas the K overlayer was disordered. However, both Na- and K-covered surfaces exhibited a metallic tunneling I-V (current-voltage) curve when a first layer by room temperature deposition was formed. Cs deposition and annealing induced the formation of a zigzag chain in which atoms were arranged in a 2X2 periodicity, but the overall surface was disordered.