Many plant leaves, notably of the Lotus flower, use a self-cleaning mechanism based on super-hydrophobicity of the leave surface. The principles and design strategies of super-hydrophobicity are presented and discussed in terms of surface features that lead to high contact angles and low roll-off angles. The thermodynamics of transition from the Wenzel regime to the Cassie-Baxter regime is discussed. In addition, the consideration of the low roll-off condition in terms of the low wet area condition is presented. Emphasis is put on the question of the applicability of super-hydrophobic surfaces to under-water systems.