producing increasingly complex patterned films and miniaturized devices, which are required in modern information and communication technologies. [1] In addition to semiconductor device fabrication, a host of applications for patterned thin films and structured polymeric systems have been identified over the last two decades in areas including optics, energy harvesting, biotechnology, and medicine. [2] Our ability to structure polymeric materials has grown tremendously and the palette of materials and techniques to achieve a quick and reliable production of patterns with increasing resolution is quite extensive. This boom has been largely sustained by the development of specific photonic technologies, such as advanced light sources or spatial light modulators, photosensitive materials, and, more importantly, our growing knowledge of the interaction between light and matter. Indeed, the ability to control a chemical reaction with light in a remote fashion is a powerful concept. Light irradiation can initiate a photochemical process that otherwise would not occur at ambient conditions. This property lies at the origin of the temporal control afforded by simply turning the light source on and off. Moreover, patterned illumination or the use of focused light beams enables control of not only when, but also where a photoinduced transformation occurs. The photochemical processes occurring in many polymers and resists can be effectively manipulated by fine-tuning the intensity and frequency of light. However, these are just two of the primary properties of light, and there are specific photoresponsive systems that enable the generation of ordered structures by adjusting the polarization of light, in addition to intensity and frequency.In this review, an overview of photochemical reactions, photosensitive polymers, and light-based structuring techniques is provided. As this field is too broad to be covered in depth in one single article, some recent examples and applications, as well as emerging opportunities and trends, are highlighted throughout the discussion. Section 2 is focused on light-induced reactions and photosensitive materials. The fundamentals of photochemical reactions and their implementation in polymer science have been overviewed in many excellent books and reviews. [3] In Section 2, rather than being comprehensive, emphasis is given to those photochemical processes lying at the basis of those structuring techniques that are later discussed in Section 3. Some of these techniques, including conventional mask lithographies, interference lithographies, direct laser writing (DLW) with one-or two-photon processes, or stereolithographic techniques, make use of light as a structuring tool.There is an additional group of techniques where light is employed to set a structure created beforehand through a Over the last few decades, the demand of polymeric structures with welldefined features of different size, dimension, and functionality has increased from various application areas, including microelectronics, ...