More recently, achievements that emerge from the "traditional" chemical engineering framework are based on the application of a new approach known as process intesification. Scientists and engineers from renowned universities and research centers are intensively working on the discovery of new process equipment and techniques. This represents a major step forward compared to the existing knowledge of chemical process plants and enables the development of integrated, safer, more energy-efficient and environmentally-more acceptable technologies. Thanks to the strong development of science and the growing interest of the public in this area, the process intensification begins to emerge as a new scientific discipline within chemical engineering. This work gives an introduction to the issue of process intesification. Here is a shorter overview of the historical development of process intensification as a scientific discipline and isolated examples of such approaches that have resulted in advanced photoreactors for water and wastewater treatment are illustrated. An overview of the conventional performance of photocatalytic reactors as well as of advanced photocatalytic reactor performance that were obtained by applying process intensification methodology is presented. Research in this area is a major challenge with regard to the ability to apply such systems to address global environmental problems, primarily those related to water treatment, wastewater, including the removal of persistent organic compounds, micro-pollutants and pathogenic microorganisms from water resources.