The Beckmann Rearrangement is a ring expansion rearrangement of an oxime =N-OH on a ring. It is a facile reaction that is able to produce even 7-membered ring lactams.
If you start with a cycloalkanone and add hydroxylamine, you get an imine with a hydroxyl group on it (an oxide). Tosylation or even acid catalysis is sufficient to cause the adjacent α-carbon to migrate to the iminium Nitrogen atom, displacing the -OH group and increasing the size of the ring by one. Attack by water, either in the medium or upon workup, gives the enamine which tautomerizes to the amide. Inside a ring, the amide make the molecule a lactam.
It is very versatile as the resulting lactam (cyclic amide) can be reduced to its corresponding amine.