The Suzuki–Miyaura (SM) cross-coupling reaction is one of the most important carbon–carbon bond-forming tools available. Its advantages primarily originate from the mild and benign nature of the boron reagent that transmetallates very efficiently with palladium(II) complexes. The precise mode of this reactivity for boronic acids and organoboranes has been debated for some time, albeit centred around just two closely related transmetallation processes. With the introduction of many new boron reagents over the last two decades, which of these two general mechanisms are followed, and whether prior activation processes are necessary, have not always been apparent. For some reagents, e.g. pinacolboronic esters or borate salts, it is uncertain if prior bond-breaking activation events need to occur for transmetallation to proceed. For other reagents, e.g. organotrifluoroborate and MIDA boronate, the activation mechanisms are better documented. This chapter categorizes the activation mechanisms and reviews what is known about each specific boron reagent, including transmetallation mechanisms, hydrolytic activation and evidence for and against prior activation.