Democracy and societal participation are dynamic and transforming concepts and are affected by the ongoing technological transitions such as digitalization. Simultaneously, young people are required to operate in complex and rapidly changing environments and to navigate through realities that are distorted with misinformation and disinformation. One way to enhance societal participation for youths is to enable taking part in societal and democratic processes. This can be conducted through providing digital services that are safe, offer access to information, and by integration to governmental processes and recognition by the officials, enable having an actual effect on policies and decisions. Although various eParticipation services have been developed and studied, thorough investigations of young people's conceptualisations, and user needs and requirements for eParticipation are missing. Moreover, the specific mechanisms through which the successful eParticipation services support young people's societal participation are unexplored. The main scientific fields that this thesis contributes are computer science and social science. More specifically, in computer science, this thesis links to the research tradition of human-technology interaction (HTI), and in social science to the research of societal participation. This thesis applies quantitative and qualitative research approaches HTI, psychometrics and applied psychology, and studies on political behaviour. The studies included in this thesis were conducted in ALL-YOUTH research project (2018-2023) funded by the Strategic Research Council of Finland. Furthermore, an eParticipation platform prototype entitled Virtual Council (Digiraati in Finnish) was simultaneously developed in the project and used as an object of the research. This thesis is composed of five publications. Four publications are based on four separate empirical studies and one publication is theoretical. Altogether 467 young Finnish people participated in the studies that took place between 2018 and 2021. Surveys were conducted among 360 participants and 107 people took part in interviews. The first study aimed to provide an understanding of the young people’s conceptions of digital participation and obstacles for participation. The second study explored the user needs of young people in regard to eParticipation. In the third study, field tests of an eParticipation service prototype were conducted. Fourth study explores the significance of digital solutions in relation to societal participation during the COVID-19 pandemic. This thesis provides theoretical and practical contributions through answering the research questions: 1. What are the youths’ conceptions regarding digital societal participation? 2. What are youths’ user needs regarding digital societal Participation? 3. How can digital solutions support societal participation of youths? In theoretical perspective, this thesis elaborates on the conceptualisation of digital and societal participation and proposes a novel model entitled Citizen-centric socio-cognitive model for participation. On a more practical level, this thesis provides a set of young people’s user needs and requirements for eParticipation services: Safe discussion environments, interesting and relevant topics, enabling reciprocal interactions with officials, feedback loops, and high level of integration to governmental processes. Moreover, feature-level solutions such as easy-to-use search tools, customisable notifications and recommendations, informative dashboards and impact representations, and anonymity were considered as solutions that may enable responding to the user needs and requirements. Additionally, as a further practical contribution, this thesis presents the Virtual Council prototype. This thesis elaborates on how eParticipation services can enable and advance the societal participation of young people by lowering the threshold to participate through various activities, and by increasing the societal participation related self-efficacy of young people. Finally, this thesis explores how digitality has supported young people during COVID-19 related lockdowns by enabling working, studying, socialising, and societal participation, and how ICT skills have been a valuable factor in sustaining coping. The results enable design and development of more inclusive and enticing eParticipation services that provide for the sustainable development of societies. The model can be utilised as a framework for research of (e)Participation and applied in public and third-sector activities planning and impact assessment. Moreover, the results further advance the theoretical and empirical research in HTI, especially in the contexts of societal participation.