This book presents peer-reviewed and selected papers of the International Youth Conference on Electronics, Telecommunications, and Information Technologies (YETI-2021), held in Peter the Great St. Petersburg Polytechnic University, St. Petersburg, on April 22-23, 2021. For the third time around, the conference brings together students and early career scientists, serving to disseminate the current trends and advances in electronics, telecommunications, optical, and information technologies. A series of workshops and poster sessions focusing, in particular, on the theoretical and practical challenges in nanotechnologies, photonics, signal processing, and telecommunications allow to establish contacts between potential partners, share new ideas, and start new collaborations. The conference is held in an online format, thus considerably expanding its geographical reach and offering an even wider scope of discussion.