Nucleation usually occurs when the original system is in the metastable state. Fluctuations in a homogeneous phase result in the formation of small clusters of a different phase; for example, liquid droplets can be formed in steam. If the steam is the stable phase, these droplets are always unstable and eventually disappear. If the steam is supercooled and the droplets have sufficiently large size, the latter become stable and continue to grow, forming centres of condensation. There is a minimum, "critical" size which allows the embryo of a new phase emerging in the metastable phase to become a centre of formation for such a new phase.

Nucleation processes are classified into homogeneous (in the bulk of the parent phase) and heterogeneous (on foreign particles, interfaces, etc.).

Examples of nucleation include:

- the crystallisation process (formation of crystals from solutions or melts) begins with nucleation. If nucleation occurs in the bulk of the phase, it is called homogeneous nucleation;

- the formation of bubbles of carbon dioxide in carbonated water, after pressure reduction in the vessel. Nucleation is faster and easier on the vessel walls, interfaces or foreign particles (heterogeneous nucleation).

Nucleation plays an important role at the initial stage of thin film growth (the critical size of the island).