This chapter details the tunnelling control of reactions involving various carbenes such as methylchlorocarbene, noradamantylchlorocarbene, cyclobutylhalocarbene, and hydroxycarbene. The very rich reactivity of nitrenes arising through their generation in the coordination sphere of metals such as copper and rhodium has been described. Comprehensive theoretical calculations have been reported for the ground and excited states of fluorobromocarbene, which is an adduct active in the stratospheric ozone layer depletion. The chapter focuses on some chosen studies in which the use of carbene ligands has a particular impact on the reaction mechanism. Combined kinetic and computational studies have been conducted to determine the activation parameters for the cyclopropylhalocarbene rearrangement to halocyclobutene. Breslow-type intermediates and their exploitation in organic synthesis continue to attract much attention from the organic chemistry community. The reaction of phosphinidene (MeP) with ethylene, acetylene, and carbon monoxide catalysed by copper complexes has been studied by density functional theory (DFT) calculations.