Hydroxylamines and hydroxamic acids are polyfunctional organic molecules and several reactive intermediates can be generated from them, therefore they have found broad application in the synthesis of various nitrogen and oxygen containing heterocycles by means of nulceophilic substitution or addition, electrophilic intramolecular aromatic substitution, as well as [2+4] and [3+2] cycloaddition reactions. This chapter surveys synthesis of heterocycles from the title compounds highlighting some distinguished reaction mechanism aspects, regio- and stereo selectivity issues and experimental details. It is arranged according to the size and type of heterocycles that allows fast but also explanatory insight into synthesis of the particular cycles.