This Special Issue of the IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES (TMTT) is comprised of expanded articles from the 2020 IEEE International Microwave Symposium (IEEE IMS) held virtually on August 4–6, 2020, in response to the worldwide COVID-19 pandemic. We thank the hard work of the IMS2020 executive and technical committees that helped to rapidly pivot from plans to hold a physical conference at the Los Angeles Convention Center to the first virtual IEEE IMS conference.