Mammalian cytochrome P450 (CYP) comprise a large group of enzymes that play many important roles in the biosynthesis of steroid hormones and vitamins. In addition, they participate in the metabolism of drugs and xenobiotics. All known mammalian CYP enzymes are membrane-associated proteins, which complicated their X-ray crystallographic analysis. In recent years, however, significant progress has been made in the X-ray crystallographic analysis of mammalian CYP enzymes involved in the steroid hormone and vitamin D(3) metabolism. The knowledge from three-dimensional structures of mammalian CYP enzymes will benefit drug discovery and development.