Periostin was originally isolated as an osteoblast-specific factor that functions as a cell adhesion molecule for preosteoblasts and is thought to be involved in osteoblast recruitment, attachment, and spreading. The protein was renamed “periostin” because of its expression in the periosteum and periodontal ligament, indicating a potential role in bone and maintenance of tooth structure. Periostin has structural similarity to insect fasciclin-I and can be induced by TGF-β and Bmp2. Because tooth and periodontium development is a well-described genetic model for organogenesis governed by a reciprocal set of epithelial–mesenchymal interactions, thought to be controlled by various TGF-β superfamily members, we investigated whether periostin is present during tooth morphogenesis. Both periostin mRNA and protein expression were analyzed throughout normal tooth development (embryonic day [E] 9.5–newborn) and within both Bmp4- and Msx2-null embryos. Periostin mRNA is initially present within the E9.5 first branchial arch epithelium and then shifts to underlying ectomesenchyme. Both mRNA and protein are asymmetrically localized to the lingual/palatal and buccal side during the early epithelial–mesenchymal interactions. Periostin is also present in dental papilla cells and within the trans-differentiating odontoblasts during the bell and hard tissue formation stages of tooth development. We suggest that periostin plays multiple roles as a primary responder molecule during tooth development and may be linked to deposition and organization of other extracellular matrix adhesion molecules during maintenance of the adult tooth, particularly at the sites of hard-soft tissue interface. Developmental Dynamics 229:857–868, 2004. © 2004 Wiley-Liss, Inc.