This special issue of Environmental Science & Pollution Research consists of ten papers that had originally been presented as oral or poster communications during the fourth European Conference on Environmental Applications of Advanced Oxidation Processes (EAAOP-4, http://www.eaaop4.com) held in Athens, Greece, between 21 and 24 October 2015. This well-established series of events attracts researchers and professionals from all over the world to discuss advances and recent trends in the field of AOPs and environment-related applications. More than 200 participants of the present edition of EAAOP-4 came from 30 countries from different geographical areas including the EU (Austria, Cyprus, France, Germany, Greece, Ireland, Italy, Portugal, Slovenia, Spain, U, etc.), Mediterranean countries (Algeria, Egypt, Israel, Tunisia, Turkey), as well as Brazil, Colombia, India, Mexico, Russia, Serbia, Switzerland, and the USA. They produced a valuable body of information and knowledge, contained in more than 300 oral and poster contributions. The EAAOP conference is now mature since its first edition in September 2006 (Crete, Greece), which points out the increasing interest of the scientific community on AOPs due to the continuous and unsustainable widespread contamination of the environment, biota, and humans by persistent toxic substances, as well as their implications for health and sustainability. The fifth edition of the conference—EAAOP5—will take place in Prague, Czech Republic, in June 2017 (http://www.eaaop5.com).

The collection included in this special issue covers topics such as process integration (e.g., AOPs coupled either together or with biological processes) for the efficient treatment of agro-industrial effluents, experimental and modeling studies, solar photocatalysis in less conventional reactor setups, environmental chemistry and ecotoxicity of transformation by-products, determination of reactive oxygen species, and testing of materials with enhanced catalytic properties. All these clearly reflect the multi- and inter-disciplinary approach needed to perform research of high calibre in the field of AOPs. Indeed, it is quite rewarding to see the participation of researchers with different backgrounds working at the interface of science and engineering to understand and develop technologies for water, air, and soil remediation.

This issue was only possible with the generous contribution of the authors and the journal’s editors and editorial assistants who handled the submissions. We are also indebted to all the reviewers who helped in making this special issue an excellent reference material.