The oxidation of one molecule is accompanied by the reduction of another. If an oxidation involves the loss of two electrons, a reduction involves the gain of two electrons. The structural changes in the reduction product are often measured by whether hydrogen is gained or if a heteroatom such as oxygen is lost. This chapter reviews important organic reactions that are classified as reductions. Just as an oxidation is a reaction in which two electrons are lost, a reduction is defined as a reaction in which two electrons are gained. Structural changes used to monitor a reduction are the gain of hydrogen atoms or loss of an oxygen atom or another heteroatom. Lithium aluminum hydride is a more powerful reducing agent than sodium borohydride, so it easily reduces ketones and aldehydes.