This chapter characterizes sustainability by discussing the sustainability, ecological, economic, and social principles and environment. Systemic approaches include sub-systems, sustainable systems, and sustainability policies. The reference values provide a quantitative threshold distinguishing sustainable from unsustainable systems. Sustainability approaches (life cycle analysis (LCA), life cycle sustainability analysis (LCSA), net primary production (NPP), human appropriation of net primary production (HANPP)) and landscape, hemeroby, and naturalness indicators provide a quantitative assessment of sustainability.