Carbenes, thought of only as transient species for a long time, have become ubiquitous in organometallic chemistry. Their interaction with a metal center, which allows for their classification as a function of the nature of the carbene-metal bond, has inspired the investigations of many research groups in every area of chemistry, from physical chemistry to organic synthesis. In this Review, we intend to give a general overview of carbenes in a broad sense, discussing singlet and triplet carbenes with all variations within these families. More precisely, we describe here, for Fischer- and Schrock-type carbenes, N-heterocyclic carbenes and non-stabilized ones, the different synthetic routes to both the immediate precursors of carbenes and their metal complexes. Additionally, their steric and electronic properties are discussed in the light of both experimental and theoretical studies.