a review is given on the use of specific biorecognition elements (such as enzymes or optically active receptors) in optical sensing of chemical species. in enzyme-based sensors various options exist' depending on which species is immobilized, assays for substrates (such as glucose, lactate or cholesterol), enzymes (such as carboxylesterase) or inhibitors (such as organophosphates) can be designed. respective examples are given. it is discussed how the intrinsic optical properties of certain enzymes or metabolites can be utilized for sensing purposes. in contrast to enzymes which 'digest' their substrates, the use of optically active receptors which do not metabolize their target molecules offers a scheme for specific recognition of substrates by a receptor. if the receptor is enantio-selective, a fairly specific recognition of enantiometers of biogenic amines becomes possible. in contrast to enzyme-based sensing where steady-state response is a result of kinetic equilibration, this type of substrate binding results in thermodynamic equilibria. specific examples are given for each sensor type, and their respective limitations are discussed