These are single-use optical sensors based on absorption, reflection, luminescence, chemiluminescence, chemiluminescence or colour measurements. The recognition system is based on both chemical and biochemical reactions and processes and has been applied to species such as nitrates, nitrites, heavy metals, alkalis, alkaline earths, amines, lactate, cholesterol, etc. Among the advantages of these sensors are their short analysis time, ease of operation, low price, in-situ results, absence of liquid reagents, possible use by unskilled personnel, no need for sample transport to the laboratory and absence of waste. Typical applications are: a) biomedical information, environmental information; b) safety in the field or factory; c) screening of critical samples; d) process monitoring by in-situ measurements; e) design of analytical strategies using rapid estimation of analyte levels; f) in-situ analysis of labile samples; and g) rapid decision making in critical situations.