Crystallography has played a fundamental role in the development of knowledge of vitamin B₁₂ chemistry and biochemistry. This role started in 1957 with the landmark crystal structure of vitamin B₁₂ or cyanocobalamin (CNCbl), which allowed the upsurge of vitamin B₁₂ chemistry, and continuing with the crystal structure of several B₁₂-dependent enzymes in 1994, which has contributed to the B₁₂ enzymology. Finally, the recent X-ray structure of B₁₂ transport proteins has furnished the structural basis for investigation of Cbl transport in mammals and for designing bioconjugates for tumour imaging and delivery of anti-tumour agents to proliferating cells. The studies on simple B₁₂ models, the well known cobaloximes have complemented those on the more complex cobalamins and B₁₂-based enzymes, with particular reference to the properties of the axial L-Co-X moiety, which are relevant to the same fragment in cobalamins. The first section summarizes some relevant structural results of cobaloximes. The second section will outline some structural properties of cobalamins. The third section is a short outline of the structure of the B₁₂ dependent enzymes. The fourth section is devoted to the structure and function of B₁₂ transport proteins. The last section summarizes the available structural results of B₁₂ bioconjugates. The aim of the present review is to complement the previous reviews on these subjects with the most recent results and to reassess some previous aspects, which were scarcely stressed. It also aims to furnish a general overview of B₁₂ structural chemistry from models to proteins for readers who would like to approach this field. Buy this Article