The increasing global health concerns due to the surge in urbanization, increasing water contamination, and increasing world industrialization are driving advanced oxidation technologies (AOTs) market globally.

This special issue of Environmental Science and Pollution Research contains a selection of 51 papers presented at the 4th Iberoamerican Conference on Advanced Oxidation Technologies (IV-CIPOA), which was chaired by Dr. Vítor Vilar, Prof. Dr. Carlos A. Martínez-Huitle, and Prof. Dr. Elisama Vieira dos Santos held in Natal City, Brazil, from 18 to 22 November 2019.

The history of CIPOA started in the year 2013. Since then the meetings have been held every 2 years. The first meeting was held in Recife (Brazil), the second in Belo Horizonte (Brazil), and later in Guatapé (Colombia) and Natal (Brazil). The number of participants has been around 200, reaching a maximum of 230 participants in Belo Horizonte.

The conference program included 12 plenary lectures, 84 regular oral presentations, 36 short oral communications, and 215 poster contributions, in the field of ozonation, advanced oxidation processes (AOPs), electrochemical advanced oxidation processes (EAOPs), and their integration with other physical/chemical and biological processes.

The present collection includes articles dealing with (i) non-photochemical AOPs in homogeneous phase (ozonation, ozonation with hydrogen peroxide, Fenton, electro-oxidation, electro-Fenton, etc.); (ii) photochemical AOPs in homogeneous phase (ionizing radiation, UV/H2O2, UV/persulfate, UV/O3, UV/O3/H2O2, photo-Fenton, electro-photo-Fenton;, etc.); (iii) non-photochemical AOPs in heterogeneous phase (catalytic ozonation, electrocatalysis, etc.); (iv) photochemical AOPs in heterogeneous phase (photocatalytic ozonation, photocatalysis, photoelectrocatalysis, etc.); (v) integration of AOTs with coagulation/flocculation, biological oxidation, adsorption, etc.; (vi) contactors/photoreactors configurations (bubble columns, tubular photoreactors, solar collectors, etc.); (vii) AOTs applied to the treatment of contaminated soil, water, and air; and (viii) photocatalysis for CO2 reduction into valuable fuels.

The guest editors would like to thank all the authors for the innovative scientific contributions to this special issue and the reviewers whose comments and suggestions were extremely important to achieve high-quality papers, as well as the institutions and companies that sponsored the IV-CIPOA meeting.

We also thank ESPR Editor-in-Chief Philippe Garrigues and the editorial assistants Fanny Creusot and Carmina Joy Cayago for their assistance/help/support in the preparation of this special issue.

Vítor J.P. Vilar wishes to thank the FCT Individual Call to Scientific Employment Stimulus 2017 (CEECIND/01317/2017) and the financial resources provided by Associate Laboratory LSRE-LCM - UIDB/50020/2020 - funded by national funds through FCT/MCTES (PIDDAC). Financial supports from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq–306323/2018-4, CNPq–312595/2019-0; CNPq–439344/2018-2) and from Fundação de Amparo à Pesquisa do Estado de São Paulo (Brazil) (FAPESP 2014/50945-4 and 2019/13113-4) are gratefully acknowledged by Elisama V. Dos Santos and Carlos A. Martínez-Huitle.