**Abstract**

Boron (B), an element that is present in ultratrace amounts in animal cells and tissues, is expected to be useful in many scientific fields. We have found the hydrolysis of C–B bond in phenylboronic acid-pendant cyclen (cyclen = 1,4,7,10-tetraazacyclododecane) and the full decomposition of *ortho*-carborane attached with cyclen and ethylenediamines in aqueous solution at neutral pH upon complexation with intracellular metals. The change in the chemical shift of the 11B signals in 11B-NMR spectra of these boron-containing metal chelators can be applied to the magnetic resonance imaging (MRI) of metal ions in solutions and in living cells. More important applications of B would be boron neutron capture therapy (BNCT) based on the nuclear reaction between 10B atoms and thermal neutrons, yielding 4 He2+ (α) and 7 Li3+ ions, which destroy 10B-containing cancer cells. The design and synthesis of new BNCT agents based on sugars and macrocyclic polyamines and their Zn2+ complexes are also introduced in this review.

**Keywords:** boron-10 (10B), boron-11 (11B), magnetic resonance imaging, metal probes, decomposition reactions, carborane, boron neutron capture therapy, macrocyclic polyamines, sugars
