Water, being a universal solvent, makes up to 70% of the weight of living organisms. The amphoteric nature of water is explained by Lewis and Bronsted acidbase theory. Hydrogen bonding is crucial in many chemical processes. It is accountable for having water's unique solvent capabilityand also for hydrophobic effect. This chapter discusses in detail the hydrogen bonding between neighbouring water molecules and theirrole in physical and chemical changes in the water molecule. It also discusses the relationship between pressure and temperature in water via a phase diagram.