This report was produced within COST Action CA21150 âParental Leave Policies and Social Sustainability (Sustainability@Leave)â, Working Group 2, and provides a systematic assessment of knowledge gaps in European research on parenting leave and social inequalities. Parenting leave policies constitute a key pillar of social sustainability, with demonstrated relevance for gender equality, labour market participation, health, and child well-being. While European research on parenting leave has expanded significantly over recent decades, the available evidence remains fragmented and uneven. Drawing on a narrative review of more than 400 publications from 24 European countries, this report maps which dimensions of inequality have been most extensively studied and identifies where substantial blind spots persist across countries, social groups, and levels of analysis. The review shows that research has focused predominantly on gender, particularly mothersâ employment trajectories and fathersâ take-up of leave. By contrast, other crucial dimensions of inequality, including health, disability, well-being, citizenship, ethnicity, non-standard employment, and diverse family forms, remain marginal in the literature. In addition, most studies implicitly centre on parents in stable, standard employment, resulting in limited evidence on the experiences of precarious workers, the self-employed, migrants, and low-income families. These biases restrict the capacity of current research to evaluate whether parenting leave policies reduce inequalities broadly or primarily benefit already advantaged groups. The report makes a central contribution by systematically linking policy design features (eligibility rules, benefit levels, individualisation, flexibility) with policy outcomes (leave take-up and longer-term inequality effects) and by highlighting the need for more intersectional, multi-level, and comparative research. It demonstrates that although a gender lens remains central to parental leave policies, future research should adopt a broader perspective to examine how multiple inequalities intersect. Strengthening data infrastructures, improving the measurement of leave use, and integrating qualitative and quantitative approaches are essential for advancing policy-relevant evidence. Policy recommendations emphasise the importance of inclusive parenting leave designs that accommodate diverse employment trajectories, ensure adequate income replacement, and support individual entitlements for both parents. The report also underlines the need for systematic monitoring of the EU WorkâLife Balance Directive, not only in terms of legal compliance but with regard to its distributional effects across social groups and countries. By identifying where evidence is missing, this report provides a strategic agenda for future research aimed at improving the inequality-reducing potential of parenting leave policies in Europe.