Systemic as well as localized skin diseases modify the molecular composition of human skin. Changes in skin chemistry have been observed in diseases such as cancer, psoriasis, eczema, diabetes, and atherosclerosis. Skin chemistry, represented by an enormous wealth of disease biomarkers including lipids, structural proteins, inflammatory mediators, nucleic acids and small molecules, therefore, can serve as a "window to body's health". Various methods including tape-stripping, iontophoresis, microneedles and ultrasound, among others, are being developed to access skin biomarkers and understand skin's detailed molecular composition. This information provides opportunities to diagnose various diseases and their response to therapeutic treatments. This review provides an overview of such diagnostic and theranostic opportunities.