Manganese(III) mediated reactions of unsaturated systems with carbonyl compounds are discussed in this review which focuses mainly on publications of the last decade. The process on the whole is highly selective both at the initiation and functionalization steps. Manganese(III) acetate generates α-oxo- and α,α-dioxoalkyl radicals by regioselective oxidation of carbonyl compounds, such as, aldehydes, ketones, acids, diketones, keto esters, and diesters. The functionalization step, which consists of the introduction of α-oxo- and α,α-dioxoalkyl moieties into multiple bond containing substrates, occurs in most cases with high regio-, chemo- and stereoselectivities. The extensive exploration of this field has uncovered its huge synthetic potential and resulted in numerous novel approaches to different classes of organic compounds. Both inter- and intramolecular versions are discussed, including for the latter mono and tandem cyclizations. The first successful examples of the syntheses of natural products having diverse biological activity complete the discussion to demonstrate the maturity of the field and its excellent prospects. 1. Introduction 2. Intermolecular Reactions 2.1. Alkenes 2.2. Alkynes 2.3. Alkadienes 2.4. 1-Alken-3-ynes 2.5. 1,3-Alkadienes 2.6. Arenes 3. Intramolecular Reactions 3.1. Mono Cyclizations 3.2. Tandem Cyclizations 4. Natural Product Syntheses 5. Methodology 6. Conclusion