Hydrogen bonds are formed, when two atoms are "sharing" a hydrogen atom. The hydrogen is often donated by an alcoholic or amino group and accepted by the free electron pair of another atom (e.g. oxygen or nitrogen). Hydrogen bonds are weak; the participating bonds have to share a common axis to be most effective.

In spite of their weakness, hydrogen bonds are important in particular when regarding biochemical structures and processes. They are responsible for a part of the uncommon properties of water and have a strong impact on the structure of various biological macromolecules.