The olefin ring-closing metathesis reaction catalyzed by ruthenium and molybdenum complexes has been employed in the syntheses of carbocycles, heterocycles, supramolecular compounds, and in tandem metathesis reactions. Chiral ruthenium and molybdenum catalysts have provided high enantiomeric and diastereomeric excesses in ring-closing metathesis reactions. Many of the unsuccessful medium- and large-sized ring formations which were unsuccessful by traditional methods, such as the intramolecular Wittig, Horner–Wadsworth–Emmons, and Julia–Kocienski reactions, can now be formed by olefin ring-closing metathesis reactions. Ring-closing metathesis has been a key step and sometimes the only successful method for the synthesis of numerous natural products and drug discovery targets. The objective of this chapter is to provide an updated, comprehensive coverage of the literature of the olefin ring-closing metathesis reaction and related processes. Key mechanistic points are summarized.