“Adsorption precursor” has been regarded as an important concept to understand gas-surface interactions since 1930s. In recent years, the adsorption precursors have been directly observed using scanning tunneling microscopies and surface spectroscopies. Some of the adsorption precursors have been found to contribute to chemical reactions with surface adsorbates. In this review article, recent studies on adsorption precursors and their contribution to surface reactions are introduced.