This article describes the importance of the surface/interface characteristics of Biomaterials. The interactions between material surface and living body (protein, cell and tissue) are reviewed from various points of view. Basic principles for protein adsorption, cell adhesion and phagocytosis of foreign materials can be obtained by surface energetic analysis. The surface modifications by grafting of water-soluble polymer, immobilizing biological substances and attaching bone component etc. are useful for improvement of the material's biocompatibility. To develop novel artificial bodysubstituting materials, the improvement of surface/interface biocompatibility is essential.