Given the importance of gender equality to international economics and sustainable economic and social development, this dissertation addresses the gender imbalances in senior company board decision-making positions. This furthermore aims to identify the potential impacts of the 33.3% gender quota for supervisory and management boards approved in Portugal in August 2017. Consequently, this enables a deeper understanding of the conditions necessary for gender quotas to bring about sustainable gender balances in boardrooms and alongside changes in the corporate culture. This dissertation applies interpretative data analysis from the European Institute for Gender Equality (EIGE) and semi-structured interviews with representatives of nine PSI 20 firms (the 20 largest companies listed on the Portuguese Stock Exchange). The goals are thus to study the glass-ceiling existence and constraints within the Portuguese corporate context, perceptions on quota outcomes and legitimacy, and the perceived impacts of women on firm performances.The results of this study demonstrate how quota acceptability is independent of gender as there were no verified gender patterns among the responses obtained; that cultural factors and gender role expectations influence board dynamics and facilitate male access to board positions; and, the Portuguese quota requirement may successfully trigger gender parity in the boardroom due to the existence of a sanctioning mechanism for noncompliance and a strong record of path dependency (earlier legal initiatives). However, findings are inconclusive as regards any possible structural change in corporate mentality and culture as respondents remain divided about the influence of gender diversity on firm performance, the legitimacy of quotas and their effects on meritocracy.