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 Trac.