The expansion of N-heterocyclic carbenes as ligands for transition metals to mesoionic ligand systems has led to the discovery of a wide range of non-classical carbene-type ligands. These non-classical carbene-type ligands are characterised by a significantly lower heteroatom stabilisation of the (putative) free carbene, a situation that also affects the ligand donor properties and hence the reactivity of the coordinated metal centre. In consequence, a number of attractive transition metal-catalysed processes have been disclosed in recent years, predominantly in the areas of cross-coupling, hydrogenations, and olefin metathesis reactions.