In the last few years the expansion of industrial activity has had a considerable impact on environmental air and water pollution. Our increasing knowledge of pollution and its effects has heightened awareness of the importance of good water, soil and air quality and led to the organization of regulatory environmental protection programmes all over the world. These programmes include: - Treatment of hazardous waste; - Detection of leakage of fuel or other chemicals into the environment; - Pollution monitoring of underground, surface and sea waters; - Evaluation of pollution at short and long distances from major factories and other polluters. Monitoring requires the continuous quantification of a great number of products or analytes at many different sites [1], and thus the work and cost involved is enormous. Traditional analysis, which implies the problems of representative sampling and subsequent sample stabilization, transport to the laboratory, preparation for analysis, measurement etc., is extremely time consuming and incompatible with the continuous monitoring of certain analytes in environmental analysis. Sampling is the most critical of these steps because of the difficulty of obtaining a representative sample. Furthermore, the stabilization of some analytes, such as sulphide, is not always an easy task and special care must be taken. Some of these problems could be overcome by performing in situ analysis which would obviate the need for a representative sample and avoid the risk of changes in the chemical form and concentration of the analyte during transport and sample pretreatment. A sensor is a device that is able to indicate continuously and reversibly the concentration of an analyte or a physical parameter [2]. If the signal originates from a chemical or biochemical reaction, the sensor is a chemical or a biochemical sensor. If a physical property is evaluated the sensor is considered a physical sensor. Sensors have made a substantial and important contribution to in situ pollution monitoring. Only those based on the use of optical fibers allow spectroscopic analysis at the site of interest. They allow the analyst to bring the laboratory to the sample instead of the sample to the laboratory as is usually the case, and thus their use may overcome or minimize the above-mentioned problems of analyte instability. The recent development of microelectronics (digital computing and semiconductor technology) and optical fibers has permitted notable progress in the determination of several analytes of industrial and environmental importance. Several reviews and books on this topic can be found in the literature [2-8]. Chemical sensors based on optical fibers are called optrodes (optical electrode) or optodes (from a Greek word for optical path). The use of fibers as simple light pipes to transmit spectroscopic information to an instrument detector is the easiest way of remote sensing. However, as the number of analytes of interest which are naturally endowed with photometric or luminescent properties is low, an unaided optical fiber is almost impractical as a sensor. A more useful type of fiber optic chemical sensor (FOCS) incorporates a reagent phase, enabling interaction with the analyte of interest. An ideal FOCS should allow in situ reversible determination of very low concentrations of contaminants in the environment. The main advantages of FOCS over other kinds of sensors can be summarized as follows: They allow in situ determination and real-time analyte monitoring; They are easy to miniaturize because optical fibers have very small diameters; They are fairly flexible: optical fibers can be bent within certain limits without damage; They can be used in hazardous places and locations of difficult access because of the ability of optical fibers to transmit optical signals over long distances (between 10 m and 10000 m); Multielement analysis is possible using various fibers and a single central unit; They normally permit non-destructive analysis; Optical fibers can carry more information than electrical cables; Probes are often easy and inexpensive to build. They also have the following disadvantages: The number or reversible reactions is very limited, so in many cases probes have to be regenerated after use; Commercial accessories for optical fibers are not standard items; The properties of the indicator may vary when it is immobilized; They usually have lower dynamic ranges than electrodes; In some cases the concentration of the immobilized indicator is unknown and two optodes prepared similarly can have different analytical characteristics; The sensor life-time is limited. Many papers and reviews dealing with the development of FOCS have been published in the literature but only a few have reported applications to environmental monitoring [8]. Therefore, this field is still in an embryonic state and major problems, such as reagent immobilization, the paucity of specific reversible reactions etc., need to be solved. The key to constructing a successful FOCS is selecting an appropriate chemical reaction that has the specificity, sensitivity and stability required to determine the analyte in the environment. Once the reaction has been chosen, an appropriate support and means of measurement have to be selected.