Contents I. Introduction 793 A. Ketene Generation 793 B. Tether Length 794 C. Effect of Substituents on the Alkene 794 D. Reaction Conditions 794 II, Substituents on the Ketene 794 A. Aldoketenes 795 1. Thermal Cycloadditions 795 2. Photochemical Addition of Aldoketenes 795 to the Double Bonds of u./S-Unsaturated Carbonyl Compounds B. Ketoketenes 796 C. a,/i-Unsaturated Ketenes 796 1. Type III Cycloadditions of 797 «,/3-Unsaturated Ketenes 2. Type II Cycloadditions of 799 a,d-Unsaturated Ketenes 3. Type I Cycloadditions of 801 a\ij-Unsaturated Ketenes D. Cyclopropylketenes 802 E. Arylketenes 802 F. Chloroketenes 803 G. Alkoxyketenes 803 FI. cr-Ketoketenes 804 I. Sulfonylketenes 805 J. Iminoketenes 805 K. Miscellaneous Ketenes 805 L. Cycloaddition of Ketenes to Carbonyl 806 Compounds III. Keteniminium Salts 806 A. Aldoand Ketoketiniminium Salts 806 B. Alkoxyketeniminium Salts 807 IV. Cycloreversion of Polycyclic Cyclobutanones 808 V. Conclusion 808 VI. Addendum 808 VII. References and Notes 810