The title compounds constitute a unique class of heterocumulenes because of their high reactivity and tendency to di-, tri- or polymerize. The chemistry is reviewed with particular emphasis on synthetic methods, stability and reactivity in heterocyclic synthesis. 1. Introduction 2. Six-Membered Ring Isocyanates 2.1. Isocyanatopyridines 2.2. Isocyanatopyrimidines 2.3. Isocyanatopyrazines 2.4. Isocyanato-s-triazines 2.5. Heterocyclization Reactions 2.6. Some Applications 3. Six-Membered Ring Isothiocyanates 3.1. Isothiocyanatopyridines 3.2. Other Isothiocyanates 3.3. Heterocyclization Reactions 3.4. Some Applications