In this lecture an overview is presented of the quantum chemical, crystallographic and quantum crystallographic studies on molecular systems containing bonds between transition metals. X-ray diffraction experiments have been fundamental for establishing the occurrence of molecules featuring one or more bonds between metals. However, because the chemistry of transition metals breaks Lewis rules, which are normally valid for organic molecules, for a correct interpretation of the nature of this chemical bond and the implication for the stability of metal clusters was necessary. The early charge density studies were only in part able to provide answers, but it was only thanks to the combination of advanced quantum chemical methods that scientists could unveil the nature of these bonds in more details. Quantum crystallography offers a seamlessly powerful way to complete the analysis, although much information is still hidden, and it requires even more complicated investigations. The metal-metal bond is extremely elusive because it cannot be easily classified into a unique category. Instead, it encompasses almost continuously all the steps from strong covalent to non-covalent.