This volume of Progress in Heterocyclic Chemistry (PHC) is the twelfth annual review of the literature, covering the work published on most of the important heterocyclic ring systems during 1999, with inclusions of earlier material as appropriate. As in PHC-11, there are also three specialized reviews in this year's volume. In the inaugural chapter, Michael Groziak revitalizes the field of boron heterocycles, a relatively obscure class of heterocycles, but with a promising future. Heterocyclic phosphorus ylides are similarly a little known but useful class of compounds and Alan Aitken and Tracy Massil have provided a comprehensive review of them in Chapter 2. In Chapter 3 Jack Li discusses the remarkably versatile palladium chemistry in pyridine alkaloid synthesis.The subsequent chapters deal with recent advances in the field of heterocyclic chemistry arranged by increasing ring size and with emphasis on synthesis and reactions.