Matriptase is an epithelial type II transmembrane serine protease with a complex modular structure and sophisticated activation mechanism. Reduced matriptase activity in mice or humans is associated with incomplete terminal differentiation of epidermis, epidermal appendages, oral epithelium, and, likely, other epithelial structures. Preliminary evidence indicates that matriptase is part of a serine protease zymogen activation cascade that regulates epithelial cell proliferation and fate. Matriptase activity must be tightly controlled in epithelial tissues by transcriptional and posttranslational mechanisms, as matriptase dysregulation can cause embryonic demise as well as malignant transformation.