Over the past few decades, there has been an increasing interest in understanding youth political engagement. However, it has been argued that the instruments used to assess the concept often lack adequate validation, and this is important as this practice may result in biased statistical conclusions. Consequently, the main aim of this thesis is to advance the field of political participation by developing a new robust psychometric instrument to assess young people's political engagement. This PhD also sets out to critically evaluate how the construct of political engagement is currently represented in research and to propose a conceptualisation of youth political engagement, and critically examine how adequately existing research instruments measure the phenomena of young people's political engagement. To achieve the main aim and objectives of this thesis, a mixed methods approach was implemented. By using qualitative methods - focus groups - it was possible to better understand young people's perceptions of the concept of political engagement, and therefore propose a new conceptualisation of young people's political engagement. Additionally, in order to develop and validate the scale to assess the construct of political engagement among young people, a quantitative approach - using confirmatory factor analysis - was adopted. Ultimately, this thesis will help clarify current conceptual and measurement debates around young people's political engagement within the field of Political Science.