the Earth's Interior the Earth's Interior Vitamin D held at the Maastricht Exhibition and Congress Centre in Maastricht, The Netherlands from July 6 to July 10, 2003. A total of 323 registered delegates from 30 countries attended. The primary function of the Vitamin D Workshop is to organize and present scientific meetings on aspects related to vitamin D. The substantial attendance at the Maastricht Workshop reflected the continuing high level of worldwide interest in research developments related to vitamin D. The quality and diversity of the science presented at the Twelfth Workshop on Vitamin D was again of an exceptionally high level. Remarkable progress continues to be made in research areas related to the vitamin D analogs of 1alpha,25(OH)2D3, the study of the biochemistry and cell biological biological effects in a wide variety of systems (including calcium homeostasis, vitamin D metabolism, regulation of gene transcription, and action of rapid responses). Also, an important component of the Workshop Program was dedicated to clinically related topics which included presentations concerning treatment of psoriasis, the application of 1alpha,25(OH)2D3 and its analogs, in the treatment of osteoporosis and application of prostate cancer.