This thesis is focused on the study of Light Pollution (LP) in urban and protected areas of Catalonia. Light pollution is any effect generated by artificial lighting. Normally Light Pollution is associated with astronomy, but there are many other effects in natural environment, health or power consumption. The evaluation of LP can be done measuring the night sky brightness (NSB) using for example telescopes and cameras or using stand-alone devices for this purpose. One of the topics of this work is the intercomparison and calibration of these devices to improve their precision and use them to establish a LP measurement network. We have measured NSB with two approaches: extensive measurements covering a large a rea as well as permanent stations to analyze the evolution of the NSB. The extensive measurements allow us to generate maps of light pollution of an area. To do that we have defined routes and a good strategy of filtering and data selection. Wlth this method we can create light pollution maps from Sky Quality Meters measurements. With these maps we can evaluate different areas and we can confirm the good quality of the Starlight protected area of Montsec with NSB compatible with a nat ural area. In the other hand we can evaluate the effects of the near city of Lleida that affects zenithal measurements up to 20 km far from the city. We have created the Catalan Light Pollution Network with ali the specifications and methods to do a permanent monitoring of the NSB. After one year of operation we can evaluate how the lunar cycle is almost not detectable in sorne periods in urban areas or how the Milky Way could affect the measurements in really dark sites with an increase of brightness up to 0.5 magnitudes. Monitoring measurements in combination with environmental data allow us to study the effects of clouds in NSB. We have determined for the first time ever how the clouds could generate a reduction of measured NSB in pristine areas meanwhile in urban areas the same kind of clouds increases the NSB up to 2 magnitudes. In the near future with more data available any other environmental parameters could be analyzed and long term evolution of light pollution could be studied.