The Symposium on Environmental Engineering (SEE), organized in the scope of the 4th Doctoral Congress in Engineering (DCE21), was hosted at the Faculty of Engineering of the University of Porto (FEUP), Portugal, on June 28th and 29th, 2021. This event was organized in virtual mode by Ph.D. students of the Doctoral Program in Environmental Engineering (PDEA) at FEUP, with the support of their professors. A book of abstracts was published gathering all the SEE-related information (ISBN: 978-972-752-280-4). The SEE covered multidisciplinary themes, with 26 oral and 40 poster presentations comprising a wide range of topics within the United Nations Sustainable Development Goals (SDGs):▪Clean Water: Emerging Pollutants, Monitoring, and Treatment; ▪Air Quality: Emissions, Assessment and Health Safety; ▪Waste Management and Circular Economy; ▪Energy, Sustainability and Innovation: Products, Technologies, and Climate Change Mitigation. In this edition, the SEE reached a record number of abstract submissions and registrations with more than 100 participants, including a representative number of international participants from Angola, Brazil, Colombia, Cyprus, Greece, Hungary, Luxembourg, and Spain. After going through a peer-review process, 5 papers were accepted for publication in this Special Issue.