The Symposium on Environmental Engineering (SEE), organized in the scope of the 4^th Doctoral Congress in Engineering (DCE21), was hosted at the Faculty of Engineering of the University of Porto (FEUP), Portugal, on June 28^th and 29^th, 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.