Direct pulp capping is the placement of a dental material directly over exposed pulp to prevent dental pulp from dying and avoid the need for root canal treatments. Calcium hydroxide and calcium silicates/mineral trioxide aggregates (MTA) have been commonly used for direct pulp capping with great success clinically. In recent years, resin-modified calcium hydroxide and calcium silicates have been developed with the advantages of precise placement, command set, and superior physical strength. As pulp capping materials would be in direct contact with the pulp, the cytotoxicity and biocompatibility is of particular importance in order to avoid pulp irritation and maintain pulp vitality. Therefore, this review article will summarize the cytotoxicity and biocompatibility of direct pulp capping materials, particularly resin-modified materials.