An outer layer of skin, roughly 1 mm thick, provides the only barrier between the body’s soft tissues and the external world. It is the largest single organ of the body. From a medical standpoint, diseases of the skin, including cancer, psoriasis, acne, and eczema, cost the United States an estimated $38.6 billion in medical costs every year (Group 2004). In addition to disease, attempts to improve skin quality and counteract aging processes provide the foundation of the multi-billion-dollar cosmeceutical industry. Thus, there are strong medical, social, and commercial motivations for improving our understanding of the structure and function of skin.