The Matteson reaction is the nucleophilic displacement of a leaving group from the α-carbon of an alkylboronic ester. The reaction proceeds through an (α-haloalkyl)boronate complex, which decomposes via stereospecific 1,2-migration and concomitant expulsion of the leaving group. Because the displacement is stereospecific and enantioenriched (α-haloalkyl)boronic esters can be used, the reaction has become a valuable tool in the asymmetric synthesis of complex natural products. The enantioenriched (α-haloalkyl)boronic esters themselves are accessed by addition of (dihaloalkyl)lithium reagents to chiral diol alkylboronic esters. Recent developments allow access to the key α-substituted alkylboronates by the addition of configurationally stable chiral α-lithioalkyl halides, carbamates, and benzoates to simple alkylboronic esters, considerably expanding the scope of the reaction. This chapter describes the mechanism of the Matteson reaction and related processes, along with its scope and limitations, and examples of its use in complex molecule synthesis.