Mineral trioxide aggregate is a recently introduced dental material. It may be used to seal perforations, make retrograde fillings in root-end resections, seal open apices, or cap vital pulps. In this review of the literature, physical, chemical, and biologic properties of MTA are discussed. Studies have suggested that MTA provides a better seal than formerly used materials such as IRM, amalgam, and Super-EBA. Further, MTA has low cytotoxicity and excellent biocompatibility. In vivo studies demonstrated a beneficial effect of MTA on pulpal and periodontal regeneration. Although controlled randomized clinical trials are still missing, MTA appears to be a suitable material to tightly seal dental hard tissues from the periodontium, or to cap the exposed pulp.