Recently it has been revealed that alkali-metal adsorption on metal surfaces does not always imply overlayer formation. Unusual structures including substitution between alkali-metal atoms and substrate atoms are observed with development of structural determination techniques. We introduce these structures and discuss about the origin of the formation. Main part of this article is Li adsorption on Cu(001) surface. With increasing Li coverage, the surface structure of Cu(001) evolves as (2•~1),

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... (3•~3)and(4•~4) at room temperature. The (2•~1) structure consists of missing-rows in the top layer of the Cu(001) surface, in which Li atoms are located. The (3•~3) and (4•~4) structures are complex structure including substituting Li atoms and Li adatoms.