Borylene ligands (:BR) are isolobal to CO and other iconic organometallic ligands. As such, borylene ligands enjoy some parallels to these ligands, but in many ways, their chemistry is distinct. This tutorial review gives an introduction to the synthesis, properties and reactivity of the major classes of transition metal borylene complexes, including terminal and bridging examples, pseudoborylenes, and metalloborylenes (borido complexes).