A molecular sensor or chemosensor is a molecular structure that is used for sensing of an analyte to produce a detectable change or a signal. The action of a chemosensor, relies on an interaction occurring at the molecular level, usually involves the continuous monitoring of the activity of a chemical species in a given matrix such as solution, air, blood, tissue, waste effluents, drinking water, etc. The application of chemosensors is referred to as chemosensing, which is a form of molecular recognition. All chemosensors are designed to contain a signalling moiety and a recognition moiety, that is connected either directly to each other or through a some kind of connector or a spacer. The signalling is often optically based electromagnetic radiation, giving rise to changes in either the ultraviolet and visible absorption or the emission properties of the sensors. Chemosensors may also be electrochemically based. Small molecule sensors are related to chemosensors. These are traditionally, however, considered as being structurally simple molecules and reflect the need to form chelating molecules for complexing ions in analytical chemistry. Chemosensors are synthetic analogues of biosensors, the difference being that biosensors incorporate biological receptors such as antibodies, aptamers or large biopolymers.