Cyclopropenes are reagents for synthesis of heterocycles and other carbocycles via the reactions discussed in this review. 1. Diels-Alder Reactions 1.1. Reaction Conditions 1.2. Effects of Cyclopropenyl Substituents 1.2.1. Inductive Effects 1.2.2. Stereochemical Consequences 1.2.3. Rearrangements of Adducts 2. [π2+π2] Additions 2.1. Self Condensations 2.2. Crossed Condensations 3. 1,3-Dipolar Additions 3.1. Addition of Diazomethanes 3.2. Addition of Aziridines 3.3. Addition of Other 1,3-Dipoles 4. Cycloadditions Across Cyclopropenyl σ Bonds 4.1. Intermolecular Condensations 4.2. Intramolecular Condensations 5. Additions of Divalent Carbon 6. Cycloadditions Initiated By Bases