Spirocyclization is a ring-forming transformation that creates a spiro compound, a polycyclic structure in which two rings share exactly one atom and no bond. The shared spiro atom is usually tetrahedral carbon but may be a heteroatom; formation of one or both bonds to that center establishes the orthogonal or near-orthogonal ring arrangement characteristic of spiro connectivity. Intramolecular nucleophilic addition, radical addition, cycloaddition, dearomatization, and transition-metal-mediated bond formation can all produce this topology. Dearomative spirocyclization forms a spiro center while converting an aromatic or heteroaromatic π system into a nonaromatic ring, as demonstrated in electrochemical processes ^[1] and ynamide cyclizations ^[2]. The term specifies product topology rather than a single mechanism. It differs from fused-ring cyclization, where two rings share two adjacent atoms and their connecting bond, and from bridged-ring formation, where rings share two nonadjacent bridgehead atoms connected by multiple paths.