Optical fibre chemical sensors are becoming more and more competitive with traditional sensors. The high degree of miniaturization, considerable geometrical versatility, easy handling, absence of electromagnetic interferences, absence of electric contacts and the possibility of remote monitoring are only some of the advantages which optical fibres can offer in comparison with other types of sensors. Many sensors have been described for the measurement of different chemical parameters: the chemical parameter most investigated with optical fibres is doubtless pH. The first pH optical fibre sensor was described in 1980, and since then, more than sixty original papers describing different pH sensors have been published. Such interest is perfectly justified since pH detection is essential in many fields of application, ranging from the environment and medicine to industry and also process control. Moreover pH transduction can be used for the measurement of different chemical species, such as carbon dioxide, ammonia and pesticides. Notwithstanding the great number of prototypes realized in different laboratories all over the world only a few products are available on the market. A critical analysis of the state of art in pH sensing using optical fibres is described, outlining the advantages and disadvantages of the optical approach.