The chemistry of simple alkene molecules on the Si(100) surface is reviewed with the newly-produced visual presentation by theoretical calculations. The early pioneering studies by the Kyoto Group and Pittsburgh group reported the di–σ bond formation and the precursor-mediated chemisorption for acetylene and ethylene on Si(100), respectively. Thereafter, these studies have been stimulating various studies of organic molecules on Si surfaces. Our recent studies have observed the precursor states for alkene chemisorption and elucidated the microscopic mechanisms of the di–σ bond formation (cycloaddition) with the help of theoretical calculations; the site-, stereo- and regio-selective chemisorption of simple alkene molecules on Si(100)c(4×2) has been established.