Particles with different properties of their constituent parts were termed after the two-faced Roman God Janus. In general, the term Janus is used to describe composite particles, with parts different in chemical nature and/or polarity. Over the past 20 years, the term Janus was also used to describe asymmetric dendrimer macromolecules and unimolecular micelles in a solution. The main focus was on firm and stable Janus structures, which are particles of complex structure, with the asymmetry being due to the differences of surface chemical groups. It should be noted that unlike core-shell particles, where the cores do not contact with the environment, in Janus particles all components come into contact with the environment.

In recent years, more and more attention in research is given to bifunctional particles, where asymmetry is determined not only by the chemical composition of the halves, but their form as well. Such particles form a large family of asymmetric or anisotropic particles, which include, for example, particles in the form of dumbbells, a snowman, an acorn, as well as particles with one of the hemispheres having the raspberry shape.

In general, bifunctional (and similar) particles are produced by asymmetrisation of the original symmetric particles, i.e., through certain chemical or physical-chemical processes that can change the symmetry of the precursor element particles. Such approaches can be divided into four groups:

- toposelective modification of the surface (when the functional chemical groups are added strictly to the required parts of the surface);

- self-assembly using the templates;

- controllable phase separation;

- a controllable surface nucleation.

Amphiphilic Janus particles, that is those composed of a hydrophilic and a hydrophobic hemispheres, can be used to stabilize emulsions such as water in oil and oil in water. It was shown that the particles with intermediate hydrophobicity are the most effective stabilizers of oil and water droplets of sub-micron diameter, and such emulsions are stable for several years.

If the surface of a Janus particle has the chemical groups that help to localise the negative charges at one hemisphere and positive charges at the other, then such particles have a huge dipole moment, so that particles can rotate depending on the polarity of the electric field and they become remote controllable.

If those particles above are also bipolar and bichrome (two colours), they can be used in electronic displays, particularly, to create electronic paper. Janus particles can also find use in drug delivery, in catalysis, in creating sensitive detectors, etc.