Detailed investigations on the use of nickel(0)-based catalysts for intramolecular [4+4]-cycloadditions are presented. Nickel(0) complexes derived from electron-rich triarylphosphines proved to be efficient catalysts for intramolecular [4+4]-cycloadditions of an array of structurally diverse bis-dienes (10 examples, up to 78% isolated yield). The reported synthetic methodology leads to cis-eight-membered fused [6.3.0] bicyclic compounds as well as trans- or cis- eight-membered fused [6.4.0] bicyclic systems. Computational studies on the stereo-determining step of the reaction in combination with experimental results demonstrated that the stereochemical outcome is dictated by the length of the chain linking the two diene units and the geometry of the C=C double bonds of the substrates.