Heterocyclicmoleculesplayasigni?cantroleinlifeprocessesandhaveplayed a major rolein industrial developments of the last century, for instance in the ?eld of dyes, pharmaceuticals, pesticides, polymers etc. They comprise not onlysomeofthemostinterestingandbiologicallyimportantnaturalproducts like alkaloids, carbohydrates, nucleic acids, and antibiotics but include many practical drugs and a large segment of known synthetic organic compounds. Hencescientistshavedevotedagreatamountofeffortto?ndoptimalsynthetic approachesto a variety ofheterocyclic compounds. Amongthemostsuccessfulandselectivesyntheticprocessesarecycload- tionreactions, since theyinvolvesimultaneous orsequential formationoftwo ormorebondsoftenwithahighdegreeofstereoselectivityandregioselectivity. Forinstance, 1,3 dipolar cycloadditionsareelectronicallyequivalent to Diels- Alder reactions and are among the most common 5-membered ring-forming systems. In addition they usually proceed with a high degree of stereo- and regio-control.It is therefore, not surprising that synthesis of many important classesofheterocycles, includingthoseofusefulbiologicallyactivemolecules, haveutilized cycloadditionsteps intheir formation.Furthermore, manyhe- rocyclesserve as intermediates in the synthesis of polyfunctional molecules. In this volume we present ?ve selected contributions by well-known - thors, each an authority in his ?eld. The ?rst chapter deals with construction ofisoxazolines(dihydroisoxazoles)via1,3-dipolarcycloadditionsofnitronates or of nitrile oxides generated from nitroalkanes. This includes inter- as well as intramolecular processes. Many of these heterocycles possess important synthetic and biological properties and are shown to lead to stereo- and - gioselectiveintroductionofmultifunctionalmoleculessuchasaminoalcohols, ?-amino acids, aldols, nitriles, and others.