This chapter discusses the chemistry of 1,3-ditungstacyclobutadienes. The metallacycles have enjoyed a pivotal position in recent advances in organometallic chemistry. The investigations by Whitesides and coworkers into the thermal decomposition of L2Pt (CH2)n compounds, where n= 4,5, and 6, were among the first to place emphasis on the importance of steric accessibility for the β–hydrogen atom abstraction reaction. X–Ray crystallographic studies have determined M–M bond distances of 2.90, 2.54, and 2.55 A for Nb, W, and Re derivatives, respectively. Addition of alkynes to the tungsten d1—d1dimers (VI) might then yield, by a coupling of the μ-CsiMe3 groups, a compound of type X. Allenes have been found to react with W2(μ–CSiMe3)2-containing compounds to give, by C–C bond formation between the tertiary carbon of the allene and the bridging carbon of one of the μ-CSiMe3 ligands, a bridging ligand, which is formally a π–allyl to one tungsten atom and a terminal alkylidene to the other tungsten. Isocyanide ligands and carbon monoxide react in hydrocarbon solvents with (PriO)4W2(μ-CSiMe3)2 to give the compounds of formula (Pr1O)(X=)W(μ-CsiMe3(μ-η2,η1-CCSiMe3)W(OPri)3. The reaction pathway interconverting a dimetallatetrahedane and two alkylidyne metal fragments still remains to be established.