In physical geography, the term hydrosphere (Greek hydro means "water") describes the collective mass of water found on, under, and over a planet's surface. The Earth's hydrosphere consists chiefly of the oceans, but technically includes clouds, inland seas, lakes, rivers, and underground waters. Hydrosphere refers to that portion of Earth that is composed of water.

It is so easy sometimes to take our hydrosphere for granted, and we seldom take the time to really think about the role that this part of the planet plays in keeping us alive. Each cell in a living organism is made up of almost 75\% water, and this allows the cell to function normally. In fact, most of the chemical reactions that occur in life, involve substances that are dissolved in water. Without water, cells would not be able to carry out their normal functions, and life could not exist.

The hydrosphere provides an important place for many animals and plants to live. Many gases, nutrients and ions are dissolved in water. The presence of these substances is critical for life to exist in water.

Hydrosphere is the home for many plants and animals. The ocean serves two main purposes in the climate system. First, it is a large reservoir of chemicals that can contribute to the greenhouse effect in the atmosphere and energy absorbing 90% of the solar radiation which hits the surface. This reservoir changes very slowly limiting how fast the climate can change. Second, it works with the atmosphere to redistribute the energy received from the sun such that the heat in the topics, where a lot of energy is received from the sun, is transferred toward the poles, where heat is generally lost to space.