The oxygen atom of the carbonyl group in aldehydes and ketones can be replaced by hydrogen by heating the semicarbazone, the hydrazone, or the azine in the presence of an alkaline catalyst. This reaction is known as the Wolff-Kishner reduction. Two slightly different variations of the method were discovered independently by Kishner in 1911 and by Wolff in 1912. Though the Kishner method has the advantage of avoiding the necessity of a sealed tube, the Wolff method has been modified to obviate both this necessity and that of isolating the intermediate carbonyl derivative. This reduction is the subject of this chapter.