Cholesteric liquid crystals (ChLCs), also known as chiral nematic liquid crystals, are a supramolecular assembly and a subclass of liquid crystal characterized by their chirality. Contrary to achiral liquid crystals, the common orientational direction of ChLCs (known as the director) is arranged in a helix whose axis of rotation is perpendicular to the director in each layer. ChLCs can be thermotropic and lyotropic. ChLCs are formed from a variety of anisotropic molecules, including chiral small molecules and polymers. ChLCs can be also formed by introducing a chiral dopant at low concentrations into achiral liquid crystalline phases.

Examples of ChLCs range from scarab beetle shells to liquid crystal displays. Many natural molecules and polymers spontaneously form the cholesteric phase. ChLCs have been used to manufacture products ranging from smart paints to textiles to and sensors. Scientists often employ biomimicry to develop ChLC-based materials inspired by natural examples.

Cholesteric liquid crystals (ChLCs) have a history dating back nearly 150 years. In 1888, the first liquid crystal — cholesteryl benzoate, a thermotropic ChLC   — was discovered by Austrian botanist and chemist Friedrich Reinitzer. Although he initially believed that cholesteryl benzoate consisted of aggregates of tiny, flowing crystals, he was confounded by the presence of two melting points. The first transition (around 145-146 °C) corresponded to a phase transition to a liquid state that possessed vibrant colors, and the second high-temperature melting point (178-180 °C) changed this cloudy liquid to a clear melt. He also discovered that this process was fully reversible. These discoveries Reinitzer reported in what is recognized as the first paper on liquid crystals.

Reinitzer was a close collaborator with Otto Lehmann, along with whom Reinitzer is considered the "father" of liquid crystals. Lehmann was the inventor of the first hot stage microscope capable of studying the thermal properties of thermotropic ChLCs, which he created in 1876. One of the key features of his microscope was crossed-polarizers. Polarized light microscopy remains highly important in the study of liquid crystals, including ChLCs. While Reinitzer quickly lost interest in the substances, Lehmann continued studying his apparently "flowing crystals" on his hot stage microscope, realizing they exhibited orientationally-dependent, vibrant colors under crossed polarizers. Lehmann was the first to coin the term liquid crystal. Studies in liquid crystals soon blossomed, and in 1922 Georges Friedel created the classification system of liquid crystals still used today. In this system, he named the chiral variety of liquid crystals cholesteric, as they were discovered from a cholesterol derivative.

Liquid crystals emerged from the status of a curiosity necessitating high temperatures to function in the 1960s, with the advent of liquid crystal display technology. Although liquid crystal displays (LCDs) are typically made of nematic liquid crystals, ChLCs have been utilized in display technology. Examples include a thermal sensor range meter created by James Fergason using ChLCs in 1959, an invention which was patented in 1960. Another example is a stress-sensing card, which when applied to skin — ordinarily black — becomes blue when the wearer is relaxed and red when stressed. The technology relies on body temperature differences between relaxation and stress.

Due to their properties intermediate between pure liquids and crystalline solids, liquid crystals are known as mesogens, a name deriving from Greek for mésos, or "intermediate". The property underpinning all liquid crystals is anisotropy (directional nonuniformity, typically manifested by an elongated rodlike shape), which under appropriate conditions (ex. high temperatures and concentrations) allows for local order around a preferred axis, named the director. Cholesteric liquid crystals are no exception. Like nematic liquid crystals, ChLCs exhibit a medium-range director along which the long axis of the liquid crystals are arranged. Unlike nematics, along a twist axis perpendicular to the director, ChLCs are arranged in layers that rotate with helical pitch p, typically defined as twice the periodicity along the twist axis.

There exist two classes of liquid crystals based on the conditions under which they form: thermotropic and lyotropic. Thermotropic liquid crystals undergo phase transitions based on temperature, whereas lyotropic liquid crystalline phases transition based on concentration within a solvent, most commonly water. For example, 5CB — a classic example of an achiral nematic thermotropic LC — undergoes an isotropic-nematic transition at 308K and a nematic-crystalline transition at 252K. Similarly, poly(n-hexyl isocyanate), a lyotropic liquid crystal, undergoes the analogous isotropic-nematic transition at weight fractions ranging from 0.225 to 0.438 in toluene, depending on molecular weight of the polymer. Cholesteric liquid crystals comprise both classes. Both small molecules and polymers can form cholesteric liquid crystals. In nature, examples include DNA, chitin, cellulose, and collagen, among others.

The local ordering in both nematic and ChLCs can be characterized according to the local nematic order parameter S. This parameter is formulated according to the following equation, a rank-2 tensor: