This book includes the proceedings of the 21st International Conference on Smart Technologies & Education (STE2024). The "International Conference on Smart Technologies & Education" (STE) is an annual global meeting dedicated to the fundamentals, applications, and experiences in the field of Smart Technologies, Online, Remote, and Virtual Engineering, Virtual Instrumentation, and other related new technologies. Nowadays, online and smart technologies are the core of most fields of engineering and the whole society. Consequently, the motto of this year's STE2024 was "Smart Technologies for a Sustainable Future". The STE conference is the successor of the long-standing annual REV Conferences and the annual meeting of the International Association of Online Engineering (IAOE) together with the EduNet World Association (EWA) and the International Education Network (EduNet). In a globally connected world, the interest in online collaboration, teleworking, remote services, and other digital working environments is rapidly increasing. In response to that, the general objective of this conference is to contribute and discuss fundamentals, applications, and experiences in the field of Online and Remote Engineering, Virtual Instrumentation, and other related new technologies like Cross Reality, Open Science and Big Data, Internet of Things and Industrial Internet of Things, Industry 4.0, Cyber Security, and M2M and Smart Objects. Another objective of the conference is to discuss guidelines and new concepts for engineering education in higher and vocational education institutions, including emerging technologies in learning, MOOCs and MOOLs, and Open Resources. This year, STE2024 has been organized in Helsinki, Finland as an onsite event supporting remote presentations, from March 6 until March 8, 2024. The co-organizers of STE2024 were the Arcada University of Applied Sciences, the International Association of Online Engineering (IAOE) together with the Global Online Laboratory Consortium (GOLC), the International Education Network (EduNet), and the EduNet World Association (EWA). STE2024 has attracted 140 scientists and industrial leaders from more than 40 countries.