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Sensors supplying a change in optical properties, easily detectable with the naked eye (visual) or inexpensive equipment such as compact spectrometers (optical), are a very powerful tool to visualise a wide range of parameters, including temperature, light, pH and concentration of chemical substances. Most of these sensors rely on indicator compounds showing a change in optical absorbance (colour) or fluorescence under the influence of a certain parameter. Halochromic dyes, for instance, change colour with pH. Since the use of nanofibres improves sensor sensitivity and response time due to their large surface area to volume ratio, the incorporation of indicator compounds into nanofibres is one of the current challenges in sensor design. This chapter discusses the production of colorimetric and fluorescent nanofibrous membranes for visual and optical monitoring (Sects. 7.3 and 7.4), supplemented by some fundamental information on those sensing systems (Sect. 7.2) and some interesting applications (Sect. 7.5).