The view of Earth from space is quite mesmerizing. Since we live on the land, we may forget that over three quarters of the Earth’s surface is actually covered by water—and what a phenomenal substance it is. Without water, life as we know it would be impossible. Astronomers studying other planets and moons in the far reaches of our solar system and beyond are frequently attempting to determine if these far away worlds are capable of sustaining life or are looking for evidence that they may have sustained life in the past. What are they looking for? They are looking for telltale geologic signs of the existence, either past or present, of water. Water is an incredibly simple molecule, but its properties are fairly unique compared to most other substances you will ever encounter. In this chapter, you will investigate the structure and properties of this most amazing and essential little compound.