Two silicon-containing catalyst systems for the olefin metathesis reaction are described. The first system, WCl6/Ph2SiH2, is active for all kinds of olefinic substrates: acyclic, cyclic and functionalized olefins, and alkadienes. The second system, W(CO)(6)/Ph2SiH2, is active for the metathesis of terminal olefins, but only under UV irradiation. Upon reaction between the catalyst components, a tungsten-silylene intermediate is formed as the precursor of the active species for metathesis. The product of a [2 + 2] cycloaddition of the tungsten-silylene complex was isolated. It showed catalytic activity for the metathesis of l-hexene under UV irradiation, or at elevated temperature, when tungsten-silylene species could be generated from [W(CO)(4)](2)[mu(2)-Si(C6H5)(2)](2). This is the first example of olefin metathesis with metal-silylene activation.